Add channel for Rubocop 1.50

[dgholz]

Following on from #308, since https://github.com/rubocop/rubocop/releases/tag/v1.25.0 has been released in the meantime

Hi, we'd like to update the version of RuboCop we install to match the latest available version (1.25.0). I don't see a matching channel branch on this repo, nor a matching engine in https://github.com/codeclimate/codeclimate/blob/master/config/engines.yml#L197. Please add support for version 1.25.0 of RuboCop.

[tnir]

Issue title should be Add channel for RuboCop 1.25. @dgholz