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.

Also install `libicu-dev` on Tracis CI in order for `charlock_holmes` (a
dependency of Linguist) to install properly.