Fixed ARC compile error when using dispatch_release with iOS >= 6.0 and OS X >= 10.8

According to https://github.com/magicalpanda/MagicalRecord/issues/198 and compiler errors when setting the base SDK to OS X 10.8, manual memory management of dispatch_queues is longer needed with ARC

Please let me know if the crude preprocessor directives can be optimized or don't work. Tbh I rarely use them.