Disable codeclimate's rubocop plugin

As stated here[1], Code Climate plugins run in isolated environments and
can't install gems or use gems installed for the repositories they're
analyzing. This is a security feature that does unfortunately limit the
flexibility of plugins to use our own third party plugins.

For this reasons, the RuboCop checks will be moved to the CI.

[1] codeclimate/codeclimate-rubocop#131 (comment)