Update rubocop and dependant gems

houndci.com was giving me false warnings about things the latest
Rubocop gem doesn’t complain about.
Merge pull request #273 from blacklane/update-rubocop-gem-0.22.0

Upgrade RuboCop to version 0.22.0.
Assign filename to buffer name

* This should help us exclude files using RuboCop
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.