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
Revert "Upgrade RuboCop to 0.23.0"

This reverts commit 1e3e4200579fd567a53b417f15776c98d1e2650d.

Rubocop has changed their configuration structure which will break existing
config for all of our users. Until we find a strategy to deal with this, we
should not upgrade any higher.
Lock RuboCop to v0.22.0

* Update configuration link to point to the right tag of RuboCop
Update configuration.haml

That ain't YAML. This is YAML.
Extract CommentingPolicy policy object

* Extract `CommentingPolicy` object for logic around permitting
  Hound to comment on a line in a pull request
Add Hound logo to home page

* Add Hound SVGs to home page
* Move line on homepage to the left
* Remove old hound image

https://trello.com/c/U2GJGKlo
Merge pull request #286 from bhardin/patch-1

Update configuration.haml
Clean up Commenter and CommentingPolicy specs

* Remove specs testing logic currently tested in `CommentingPolicy`
  specs from `Commenter`
* Don't stub unused and nonexistent methods in `CommentingPolicy` specs
Rename CommentingPolicy spec to have suffix

Tighten up some lines that dont need to wrap beyond one line.
Post Travis build status to #hound

* Notify on build failure
* Notify on first success after previous failure
Add contribution section to README
Minor changes
Another minor change
Fix markdown link