Run RuboCop on all Ruby versions >= 2

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'