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.
I want to make it explicit what we are returning - the default behavior is a bit unexpected for people reading the code.  Generally, if you exit early, you do something explicit (log, return error, or display response), vs just returning.  Also, since we're returning no content, returning 204 is appropriate as long as it doesn't break GitHub.