Upgrade RuboCop to 0.23.0

* Change Rubocop reference to RuboCop
* Prefix configuration according to new syntax
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.
Merge pull request #275 from Koronen/vk-linguist

Use Linguist to determine file language
Refactor ModifiedFile to CommitFile

Generalize the name to handle files that may not have been modified, like
`.hound.yml`.