Ping author when pull request is not mergeable

The message is not generic enough to work for all projects as we use 'develop' in Foreman core and master everywhere else, as far as I can recall. This would simply notify pull request authors that they have to rebase the pull request as it's no longer mergeable. 

It happens often that if a PR stays for too long in the waiting line, it becomes unmergeable, and github doesn't tell the user automatically, so it's normally us that have to remind contributors.

Comments look like this https://github.com/eLobato/foreman/pull/5

Any ideas?