Rename methods for adding comments

* I have a feature branch that implements issue commenting. This commit
  renames methods in `PullRequest` and `GithubApi` in anticipation of the
  changes on my branch.
* Rename `PullRequest#add_comment` to `PullRequest#comment_on_violation`
* Rename `GithubApi#add_comment` to `GithubApi#add_pull_request_comment`
Yes, good call. Changed in https://github.com/thoughtbot/hound/commit/1f66211e94bd16ca764f01ba7f5467f3a08abcd7.