Refactor ternaries in -swapInDisposable:
Use the same condition for retain _and_ release

This fixes analyzer warnings about newDisposablePtr potentially leaking.
Whoops, release the _existing_ disposable on return
Merge pull request #880 from ReactiveCocoa/false-leak-fix

Fix another analyzer false positive in RACSerialDisposable
Merge remote-tracking branch 'origin/master' into 3.0-development