Don't trigger builds triggered by the same event

Now any builds are triggered even if existing builds triggerd by the same
event are running.

This patch prevents to trigger build according to the below policy:

* Project has triggered/running build triggered by the same event.
* The event trigger builds has still running build.

This would fix JENKINS-24445.

Fix for JENKINS-24445

Task-Url: https://issues.jenkins-ci.org/browse/JENKINS-24445
Merge pull request #172 from rinrinne/running-trigger

Don't trigger builds triggered by the same event