Fix Swift bridging for RACQueueScheduler

This fix should enable `import ReactiveCocoa` in a Swift Playgrounds, and improve LLDB "stability" for Swift targets using ReactiveCocoa.

It turns out the swift compiler compiles with `OS_OBJECT_HAVE_OBJC_SUPPORT=0`, and this disables ARC for GCD objects. :cry:
Can you split this onto two lines, and check `_queue` for `NULL` before releasing it?