Move rubocop dependency and configuration to hound-rubocop gem

Attempt to solve #472 and #480

The gem is located at http://github.com/theodorton/hound-rubocop - I was thinking in the lines of:
- Version always matches `rubocop` version, and using a 4th decimal for patches, e.g. `0.25.0.1`
- Developers can add `gem 'hound-rubocop', group: :development` to their `Gemfile` and be sure they're using the same configuration as `hound`- no surprises when opening the PR and no headache trying to get the configuration right

I haven't published the gem to Rubygems, I think it's approperiate that someone at thoughtbot does the publishing.