Display failure status when config is invalid

* Hound users currently receive no feedback when their Ruby, JavaScript,
  and CoffeeScript configuration files are invalid. This commit
  introduces a new feature to address this disconnect:
  When a config file is empty or can't be parsed, the GitHub status will
  be changed to "failure".
* Validate repo config in `BuildRunner#run`. If config is invalid,
  create failure status and create build record.
* Add `RepoConfig#validate` and `RepoConfig#errors`
* Add `GithubApi#create_failure_status`
* https://trello.com/c/JLt3FVUb/397-use-status-api-to-tell-user-when-custom-config-is-broken
I enjoy this refactor but have some pause around a boolean method (`invalid?`) mutating the object's instance methods (`@errors`) in this case.  The only idea I can come up with to minimize this concern is to update the `repo_config.invalid?` method to simply execute `errors.any?` and then have an additional method on `repo_config` called `errors` that memoizes `invalid_style_guides.map do |style_guide|...`.  In this way, I think my concern mentioned above is reduced as well as allowing for `invalid`? to be called again and not re-run the `repo_config. invalid_style_guides` method.

Thoughts on this idea?