Disable Rails/UniqBeforePluck rubocop cop

 Rails/UniqBeforePluck seems to have some bugs 

* https://github.com/bbatsov/rubocop/issues/3122
* https://github.com/bbatsov/rubocop/issues/3148

and we had some problems in EE with that https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/425#note_12245005

See merge request !4477