Use HoundCI instead of rubocop

HoundCI is basically rubocop as a service. Excellent idea, let's try it out.

* You can configure it with a `.hound.yml` that is the normal rubocop
 configuration file.
* It will comment on the pull request – that's exactly what we want!

I removed rubocop, because everything we do on the Repo is done via a pull
request. This is part of the review process, and doesn't need to be done on
the local machine.