Display failure status when config is invalid

This PR is a continuation of https://github.com/thoughtbot/hound/pull/525.

* 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#invalid` and `RepoConfig#load_style_guides`
* Add `GithubApi#create_failure_status`
* Refactor constructor of StyleChecker to accept repo config
* Remove null constraint and presence validation for violation filename
* https://trello.com/c/JLt3FVUb/397-use-status-api-to-tell-user-when-custom-config-is-broken