Fix crash in SPTDataLoaderRequest for iOS < 7.1
Fix crash in SPTDataLoaderRequest for iOS < 7.1

We can't use OSAtomicIncrement64Barrier as we're targetting iOS < 7.1 where
the function does not seem to be properly supported / even exist.

Change-Id: Ib3ae12afbe6c9f3c874f28c18f8fbbd2028acef5

spotify/SPTDataLoader,spotify/SPTDataLoader,spotify/SPTDataLoader,iOSCowboy/SPTDataLoader,iOSCowboy/SPTDataLoader