ignore spurious UIApplicationDidBecomeActiveNotification events

If a RM app receives extra `UIApplicationDidBecomeActiveNotification` events while a test suite is in progress, the test harness will attempt to re-load the Bacon module definition (`lib/motion/spec.rb`).

Extra `UIApplicationDidBecomeActiveNotification` notifications will happen if the simulator throws up a permissions dialog (e.g. for access to the Address Book or some other iOS service).

This patch ignores any of these notifications after the first.  Bacon will only be initiated the first time.