Failing unit test to expose thread-unsafe implementation of -[RACSerialDisposable disposable].
Reimplement RACSerialDisposable with OSSpinLock rather than OSAtomic in order to fix -disposable method to be thread safe.
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Merge pull request #1456 from talko/serial

Fix lack of thread safety in -[RACSerialDisposable disposable]
Merge remote-tracking branch 'origin/master' into xcode6

Conflicts:
	ReactiveCocoaTests/RACSerialDisposableSpec.m