[EE] Upgrade Rubocop to 0.68.1
This adds Ruby 2.6 support.
The bulk of changes were generated with three commands:
bundle exec rubocop --auto-gen-config
bundle exec rubocop --a
bundle exec haml-lint --auto-gen-config
CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28046
Edited by Stan Hu