Run transitions on NSOperationQueue.mainQueue

I had some issues with a state machine stored in a thread-safe singleton class. Notifications dispatched from worker queues via [Bubblewrap's NSNotificationCenter helpers](https://github.com/rubymotion/BubbleWrap#nsnotificationcenter) couldn't trigger a state transition.

My changes makes `NotificationTransition` follow [the same NSNotificationCenter design as Bubblewrap](https://github.com/rubymotion/BubbleWrap/blob/36062f34db57d3bca1564db8f87b0aa1f046b823/motion/core/ns_notification_center.rb).

I couldn't tell if this has unintended consequences?