diff --git a/.rubocop.yml b/.rubocop.yml
index 01f9380..125d500 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -1,5 +1,17 @@
 AllCops:
   DisplayCopNames: true
+  TargetRubyVersion: 2.4
+
+## Metrics #####################################################################
+
+Metrics/BlockLength:
+  Exclude:
+    - "Guardfile"
+    - "spec/**/*"
+
+Metrics/MethodLength:
+  CountComments: false
+  Max: 15
 
 ## Styles ######################################################################
 
@@ -53,9 +65,3 @@ Style/EmptyCaseCondition:
 #   end
 Style/TrivialAccessors:
   Enabled: false
-
-## Metrics #####################################################################
-
-Metrics/MethodLength:
-  CountComments: false
-  Max: 15
diff --git a/.travis.yml b/.travis.yml
index a6d8d16..d39bea2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,12 +4,19 @@ env:
     - JRUBY_OPTS="$JRUBY_OPTS --debug"
 language: ruby
 rvm:
-  - 2.0.0
   - 2.1
   - 2.2
   - 2.3.0
+  - 2.3.1
+  - 2.3.2
+  - 2.3.3
   - 2.4.0
+  - ruby-head
   - jruby-9.1.6.0
+  - jruby-head
 matrix:
+  allow_failures:
+    - rvm: ruby-head
+    - rvm: jruby-head
   fast_finish: true
 sudo: false
diff --git a/Gemfile b/Gemfile
index e9e2b0a..f7282a3 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,23 +1,25 @@
+# frozen_string_literal: true
+
 source "https://rubygems.org"
 
 gem "rake"
 
 group :development do
-  gem "pry"
   gem "guard"
   gem "guard-rspec", :require => false
+  gem "pry"
 end
 
 group :test do
   gem "coveralls"
   gem "rspec",      "~> 3.1"
+  gem "rubocop",    "= 0.47.0"
   gem "simplecov",  ">= 0.9"
-  gem "rubocop",    "= 0.40.0"
 end
 
 group :doc do
-  gem "yard"
   gem "redcarpet"
+  gem "yard"
 end
 
 # Specify your gem's dependencies in form_data.gemspec
diff --git a/Guardfile b/Guardfile
index 7d6acb4..60f47a2 100644
--- a/Guardfile
+++ b/Guardfile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 guard :rspec, :cmd => "bundle exec rspec" do
   require "guard/rspec/dsl"
   dsl = Guard::RSpec::Dsl.new(self)
diff --git a/README.md b/README.md
index ccbc2e7..411ea8a 100644
--- a/README.md
+++ b/README.md
@@ -50,7 +50,6 @@ socket << form.to_s
 This library aims to support and is [tested against][ci] the following Ruby
 versions:
 
-* Ruby 2.0.0
 * Ruby 2.1.x
 * Ruby 2.2.x
 * Ruby 2.3.x
diff --git a/Rakefile b/Rakefile
index 158675f..028126f 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,4 +1,4 @@
-#!/usr/bin/env rake
+# frozen_string_literal: true
 
 require "bundler/gem_tasks"
 
diff --git a/appveyor.yml b/appveyor.yml
index 900265c..7b50d35 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -1,7 +1,5 @@
 version: "#{build}"
 build: off
-init:
-  - git config --global core.autocrlf true
 install:
   - set PATH=C:\Ruby23\bin;%PATH%
   - bundle install
diff --git a/http-form_data.gemspec b/http-form_data.gemspec
index bb24696..92e0d32 100644
--- a/http-form_data.gemspec
+++ b/http-form_data.gemspec
@@ -1,4 +1,5 @@
-# coding: utf-8
+# frozen_string_literal: true
+
 lib = File.expand_path("../lib", __FILE__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require "http/form_data/version"
diff --git a/lib/http/form_data.rb b/lib/http/form_data.rb
index 9179a30..73749af 100644
--- a/lib/http/form_data.rb
+++ b/lib/http/form_data.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "http/form_data/file"
 require "http/form_data/multipart"
 require "http/form_data/urlencoded"
@@ -26,7 +28,7 @@ module HTTP
   #   socket << form.to_s
   module FormData
     # CRLF
-    CRLF = "\r\n".freeze
+    CRLF = "\r\n"
 
     # Generic FormData error.
     class Error < StandardError; end
diff --git a/lib/http/form_data/file.rb b/lib/http/form_data/file.rb
index 900eddc..adcf5de 100644
--- a/lib/http/form_data/file.rb
+++ b/lib/http/form_data/file.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module HTTP
   module FormData
     # Represents file form param.
@@ -18,7 +20,7 @@ module FormData
     #  FormData::File.new "/home/ixti/avatar.png"
     class File
       # Default MIME type
-      DEFAULT_MIME = "application/octet-stream".freeze
+      DEFAULT_MIME = "application/octet-stream"
 
       attr_reader :mime_type, :filename
 
diff --git a/lib/http/form_data/multipart.rb b/lib/http/form_data/multipart.rb
index 2c75dfb..efd850f 100644
--- a/lib/http/form_data/multipart.rb
+++ b/lib/http/form_data/multipart.rb
@@ -1,7 +1,7 @@
-# stdlib
+# frozen_string_literal: true
+
 require "securerandom"
 
-# internal
 require "http/form_data/multipart/param"
 
 module HTTP
@@ -11,7 +11,7 @@ class Multipart
       # @param [#to_h, Hash] data form data key-value Hash
       def initialize(data)
         @parts          = Param.coerce FormData.ensure_hash data
-        @boundary       = ("-" * 21) << SecureRandom.hex(21)
+        @boundary       = (Array.new(21, "-") << SecureRandom.hex(21)).join("")
         @content_length = nil
       end
 
diff --git a/lib/http/form_data/multipart/param.rb b/lib/http/form_data/multipart/param.rb
index a8617de..32b9c90 100644
--- a/lib/http/form_data/multipart/param.rb
+++ b/lib/http/form_data/multipart/param.rb
@@ -1,21 +1,23 @@
+# frozen_string_literal: true
+
 module HTTP
   module FormData
     class Multipart
       # Utility class to represent multi-part chunks
       class Param
+        CONTENT_DISPOSITION = ""
+
         # @param [#to_s] name
         # @param [FormData::File, #to_s] value
         def initialize(name, value)
-          @name = name.to_s
-          @value = value
-
+          @name   = name.to_s
+          @value  = value
           @header = "Content-Disposition: form-data; name=#{@name.inspect}"
 
           return unless file?
 
-          @header << "; filename=#{value.filename.inspect}"
-          @header << CRLF
-          @header << "Content-Type: #{value.mime_type}"
+          @header = "#{@header}; filename=#{value.filename.inspect}#{CRLF}" \
+                    "Content-Type: #{value.mime_type}"
         end
 
         # Returns body part with headers and data.
diff --git a/lib/http/form_data/urlencoded.rb b/lib/http/form_data/urlencoded.rb
index 16ba9cc..6e09dc9 100644
--- a/lib/http/form_data/urlencoded.rb
+++ b/lib/http/form_data/urlencoded.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "uri"
 
 module HTTP
diff --git a/lib/http/form_data/version.rb b/lib/http/form_data/version.rb
index b2bf8cc..82a2ad4 100644
--- a/lib/http/form_data/version.rb
+++ b/lib/http/form_data/version.rb
@@ -1,6 +1,8 @@
+# frozen_string_literal: true
+
 module HTTP
   module FormData
     # Gem version.
-    VERSION = "1.0.1".freeze
+    VERSION = "1.0.1"
   end
 end
diff --git a/spec/lib/http/form_data/file_spec.rb b/spec/lib/http/form_data/file_spec.rb
index 8dd5cee..e7564b9 100644
--- a/spec/lib/http/form_data/file_spec.rb
+++ b/spec/lib/http/form_data/file_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # coding: utf-8
 
 RSpec.describe HTTP::FormData::File do
diff --git a/spec/lib/http/form_data/multipart_spec.rb b/spec/lib/http/form_data/multipart_spec.rb
index de7176a..8571fab 100644
--- a/spec/lib/http/form_data/multipart_spec.rb
+++ b/spec/lib/http/form_data/multipart_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 RSpec.describe HTTP::FormData::Multipart do
   let(:file)          { HTTP::FormData::File.new fixture "the-http-gem.info" }
   let(:params)        { { :foo => :bar, :baz => file } }
diff --git a/spec/lib/http/form_data/urlencoded_spec.rb b/spec/lib/http/form_data/urlencoded_spec.rb
index 6628c93..3279ab0 100644
--- a/spec/lib/http/form_data/urlencoded_spec.rb
+++ b/spec/lib/http/form_data/urlencoded_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # coding: utf-8
 
 RSpec.describe HTTP::FormData::Urlencoded do
diff --git a/spec/lib/http/form_data_spec.rb b/spec/lib/http/form_data_spec.rb
index 980790f..9798ab8 100644
--- a/spec/lib/http/form_data_spec.rb
+++ b/spec/lib/http/form_data_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 RSpec.describe HTTP::FormData do
   describe ".create" do
     subject { HTTP::FormData.create params }
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index f24ed16..76f5f92 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,4 +1,4 @@
-# coding: utf-8
+# frozen_string_literal: true
 
 require "simplecov"
 require "coveralls"
diff --git a/spec/support/fixtures_helper.rb b/spec/support/fixtures_helper.rb
index 0dd3498..a3ef8ba 100644
--- a/spec/support/fixtures_helper.rb
+++ b/spec/support/fixtures_helper.rb
@@ -1,4 +1,4 @@
-# coding: utf-8
+# frozen_string_literal: true
 
 require "pathname"