Auto-merge Pull Requests from bots

[philschatz]

... after all required CI Status checks pass of course 😄

The idea for this is that several repositories have dependency-checker bots that update packages and trigger Travis to run. When those all pass then we could automatically merge these types of PullRequests.

Steps

This code should run when a pull_request.created or status (passed) event occurs.

1. Ensure the PR is made by a bot
2. Check if the PR is mergeable (passes CI and has a reviewer if those are configured to be required for the master branch), is made by a bot, and get the commit await octokit.pullRequests.get({owner, repo, number})
3. Merge the PR await octokit.pullRequests.merge({...})