Only notify Mergers once all requirements are satisifed

[James-Firth]

Request from Jen via Slack:

feature request for git slackin': notify me once all the requirements are met
I often find myself forgetting about the builds I'm waiting on before merging and get side tracked

This would improve the signal-to-noise ratio for merger notifications!

• Add a new function called checkForRequirements (or something) that will check both reviews (currently handled by checkForReviews()) and ensure the Github "Checks" are all completed/ok.
• Perform the checkForRequirements on receipt of this webhook, to make sure notifications happens.
  https://developer.github.com/v3/activity/events/types/#checkrunevent