Upgrade to Rubocop 0.68.1
This adds Ruby 2.6 support.
The bulk of these changes were generated with three commands:
bundle exec rubocop --auto-gen-config
bundle exec rubocop --a
bundle exec haml-lint --auto-gen-config
EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11982
Edited by Stan Hu