Merge branch 'change-rubocop-to-2-1' into 'master'

Change target Ruby version for RuboCop to 2.1

We have to use the lowest common denominator to check the supported syntax and in our case it is Ruby 2.1. Please note that it will not help with unsupported syntax in HAML files because they are not checked by Rubocop.

See merge request !6978