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?
They're not removed, it might be a good idea though. Let me check if 

```ruby
pull_request.replace_labels(['Waiting on contributor'], ['Needs testing', 'Needs re-review', 'Not yet reviewed'])
```

would work or it'd 500 if the other labels are not available