Use branch instead of ref in Request::Approval

Pull Requests have refs on the form "refs/pulls/[pull-id]/merge", which
doesn't match the Regexp, which is the cause of #1056. This uses the
branch instead, and fixes #1056.
add one small test which covers a PR case
Merge pull request #224 from travis-ci/hh-use-branch-over-ref-in-approval

Use branch instead of ref in Request::Approval