Ping event should not trigger builds

While reviewing @bparees sample application I found out that he was triggering build with ping event, which is abiously wrong, I must have overlooked that case. So this PR extendeds integration tests with that test case and fixes it.
@bparees @mfojtik review pls.
Github doesn't really cares about response code. In that case I don't think we need to return sth. special for this, as this always returns 200 here, at the end exactly the same.