Run RuboCop on all Ruby versions >= 2

Author: sferik

Gemfile

@@ -13,14 +13,17 @@ group :test do
   gem 'addressable', '~> 2.3.8'
   gem 'backports'
   gem 'coveralls'
-  gem 'mime-types', '~> 1.25', :platforms => [:jruby_18, :ruby_18]
   gem 'rack', '~> 1.2', :platforms => [:jruby_18, :jruby_19, :ruby_18, :ruby_19, :ruby_20, :ruby_21]
-  gem 'rest-client', '~> 1.6.0', :platforms => [:jruby_18, :ruby_18]
   gem 'rspec', '>= 3'
-  gem 'rubocop', '>= 0.37', :platforms => [:ruby_20, :ruby_21]
+  gem 'rubocop', '>= 0.37', :platforms => [:ruby_20, :ruby_21, :ruby_22, :ruby_23]
   gem 'simplecov', '>= 0.9'
   gem 'yardstick'
 
+  platforms :jruby_18, :ruby_18 do
+    gem 'mime-types', '~> 1.25'
+    gem 'rest-client', '~> 1.6.0'
+  end
+
   platforms :ruby_18, :ruby_19 do
     gem 'json', '< 2.0'
     gem 'term-ansicolor', '< 1.4.0'