Remove nkf dependency.

Looks like we don't need to convert subject to utf-8 anymore with newer Rubies. Tests pass without
it.

Author: nashby

letter_opener.gemspec

@@ -12,7 +12,6 @@ Gem::Specification.new do |s|
   s.require_path = "lib"
 
   s.add_dependency 'launchy', '>= 2.2', '< 3'
-  s.add_dependency 'nkf'
 
   s.add_development_dependency 'rspec', '~> 3.10.0'
   s.add_development_dependency 'mail', '~> 2.6.0'

lib/letter_opener/message.rb

@@ -2,7 +2,6 @@
 require "erb"
 require "fileutils"
 require "uri"
-require "kconv"
 
 module LetterOpener
   class Message
@@ -88,7 +87,7 @@ def sender
     end
 
     def subject
-      @subject ||= @mail.subject.toutf8
+      @subject ||= @mail.subject
     end
 
     def to