Update GitHub status checks on pushes

[sjwl]

Similar to how pull request resources have the ability to update GitHub's statuses of a given commit (https://developer.github.com/v3/repos/statuses/), we'd like to reflect the status of a triggered job of the subsequent merge/push to master branch.

The commit hash of the final PR just before merging, does not always match the final commit hash on master branch, since we often squash and merge. As a result, sometimes the status check doesn't reflect on the commit lists off the master branch.