[REQ] Add RuboCop to Gemfile

[ackintosh]

Describe the solution you'd like

I would suggest to add Rubocop to Gemfile so that users won't need to install that globally.

group :development, :test do
   gem 'rake', '~> 12.0.0'
   gem 'pry-byebug'
+  gem 'rubocop', '~> 0.66.0'
end

cc Technical Committee: @cliffano (2017/07) @zlx (2017/09) @autopp (2019/02)

[wing328]

Sounds good to me

[autopp]

@ackintosh Sounds good to me!

[ackintosh]

Thanks for the feedback @wing328 @autopp ! I'll file a PR to close this issue later today. 💪