Differential builds with on Darwin now enabled

Hello everyone, Note that the Mac Mini builder will now build submitted Differentials as well as commits to master. Originally I was hesitant to take this step since unreviewed differentials are essentially untrusted code; however it has become clear that to keep regressions from entering the tree we will need to be a bit more proactive in testing code before it is committed. Hopefully this helps! Lastly, I'd like to take this opportunity to once again thank Futurice for providing this hardware to us. We all owe them a debt of gratitude. Cheers, - Ben