Drop legacy Ruby and Rails support

`ERB::Util.html_escape_once` is available in all supported Rails
versions

Also:
- Opt-in for MFA (security)
- Move development gems to Gemfile

Close jamesmartin#161, close jamesmartin#164

Gemfile

@@ -2,3 +2,9 @@ source 'https://rubygems.org'
 
 # Specify your gem's dependencies in inline_svg.gemspec
 gemspec
+
+gem "bundler"
+gem "pry"
+gem "rake"
+gem "rspec"
+gem "rubocop"

inline_svg.gemspec

@@ -18,13 +18,9 @@ Gem::Specification.new do |spec|
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ["lib"]
 
-  spec.add_development_dependency "bundler", "~> 2.0"
-  spec.add_development_dependency "rake"
-  spec.add_development_dependency "rspec", "~> 3.2"
-  spec.add_development_dependency "rspec_junit_formatter", "0.2.2"
-  spec.add_development_dependency "pry"
-  spec.add_development_dependency "rubocop"
+  spec.metadata['rubygems_mfa_required'] = 'true'
+  spec.required_ruby_version = '>= 3.1'
 
-  spec.add_runtime_dependency "activesupport", ">= 3.0"
-  spec.add_runtime_dependency "nokogiri", ">= 1.6"
+  spec.add_runtime_dependency "activesupport", ">= 6.1"
+  spec.add_runtime_dependency "nokogiri", ">= 1.16"
 end

lib/inline_svg/action_view/helpers.rb

@@ -22,15 +22,6 @@ def inline_svg(filename, transform_params={})
 
       private
 
-      def backwards_compatible_html_escape(filename)
-        # html_escape_once was introduced in newer versions of Rails.
-        if ERB::Util.respond_to?(:html_escape_once)
-          ERB::Util.html_escape_once(filename)
-        else
-          ERB::Util.html_escape(filename)
-        end
-      end
-
       def render_inline_svg(filename, transform_params={})
         begin
           svg_file = read_svg(filename)
@@ -60,7 +51,7 @@ def read_svg(filename)
 
       def placeholder(filename)
         css_class = InlineSvg.configuration.svg_not_found_css_class
-        not_found_message = "'#{backwards_compatible_html_escape(filename)}' #{extension_hint(filename)}"
+        not_found_message = "'#{ERB::Util.html_escape_once(filename)}' #{extension_hint(filename)}"
 
         if css_class.nil?
           return "<svg><!-- SVG file not found: #{not_found_message}--></svg>".html_safe