pin rubocop for rails

Author: JonRowe

Gemfile

@@ -60,9 +60,9 @@ if RUBY_VERSION <= '1.8.7'
   gem 'rubyzip', '< 1.0'
 end
 
-unless RUBY_VERSION >= '2.4.0'
+if RUBY_VERSION >= '1.9.3' && RUBY_VERSION < '2.4.0'
   # our current rubocop version doesnt support the json version required by Ruby 2.4
-  gem 'rubocop', "~> 0.23.0", :platform => [:ruby_20, :ruby_21]
+  gem 'rubocop', "~> 0.23.0"
 end
 
 custom_gemfile = File.expand_path("../Gemfile-custom", __FILE__)