update with JUnitRunner

Update Specs2 to 2.4.15 and add JUnitRunner to be able run test automatically from Eclipse/ Scala IDE
Ah I (maybe) already know what "nit-picking", sorry about that, I don't want to "seeking attention" or whatever. I made a pull request because my coworker that is new to Scala pull this seed from Activator, and waste his time in a day just because he can't run test in Eclipse and then I initiate to add this pull request. It's ok for me if you reject this, no hard feeling.