Add correct check for ARC support for GCD objects

The fix provided in https://github.com/soffes/sspulltorefresh/pull/16 is incorrect. As you can see in <os/object.h> (/usr/include/os/object.h), there's already a macro defined when objc support is available for GCD objects. Check for this instead. The previous way of doing it causes a memory leak when using ARC and a deployment target < iOS 6.0 because ARC support for GCD is only available after 6.0