Allow bors to look at gihtub commit status API for merge commit

Pair-programmed with @reiddraper

Instead of polling buildbot's JSON status, if you have your CI server
(not just buildbot but even something like TravisCI) posting to the
github status API, you can use the new 'use_github_commit_status_api'
boolean option to check for the commit status on the merge commit to see
if the integration commit has passed the tests.