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.
In that case the default build creation would be scattered around many plugins which is what I wanted to avoid. I figured it out that adding 3rd return parameter would make more sense than putting additional functionality into returning build vs nil. I was also considering returning special Error for that case, but again this is not error situation, it's just a signal not to trigger build.