"before" field of a push event's payload already shows the commit before the push event was sent, so we can use it as "from" field of the revision.
Fixes#18
For any event that is not of type `pull_request`, we consider there will be a `GITHBU_SHA` defined and run lint against only this specific commit
sha.
This supports not only events of type `pull` but also other events that provide us a `GITHBU_SHA`, like `check_suite`.
