Set RuboCop's target Ruby version for Hound

By default, RuboCop looks at the .ruby-version file at the project root, if any.

https://github.com/bbatsov/rubocop/blob/3d63895ea625312a8a18303f7343a19075299813/manual/configuration.md#setting-the-target-ruby-version

However, Hound only has access to the changed files in each run.

https://github.com/houndci/hound/issues/1230

So if we want Hound to stop hounding us about Ruby 2.3 features (e.g. https://github.com/otwcode/otwarchive/pull/2786#discussion_r104586828) we have to set the Ruby version explicitly in .rubocop.yml.