Use Linguist to determine file language

Using Linguist should allow us to lint Ruby source files not necessarily ending in `.rb`, such as `Gemfile`, `Rakefile`, `config.ru` etc. Leveraging Linguist should also make it easier to extend Hound with support for additional languages, since we don't have to concern ourselves with the mechanics of finding out what files belong to that language.

Does this sound like a good idea?