Configure Hound CI

[orenyk]

Now that we're implementing more JS stuff with #1572 and we want to properly use a JS linter (also see #1077), we think it would be better to use Hound CI for style checking. It's been activated for this repository; we should read through the documentation and set up a .hound.yml file for proper configuration. If it works well we could potentially remove rubocop from Travis, although if they're just doing the same checks it might be better to keep it as part of our CI build as well.