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.
We should probably write an explicit response code here - what status code does github accept for ping?  204 seems appropriate for what we're doing