Update requirements to iOS 8.0 and OS X 10.8

It looks like the Xcode project specifies a minimum or OS X 10.8 SDK, and uses iOS 6.0+ only features (strong properties with a type of `dispatch_queue_t`) but specifies iOS 8.0 for the dynamic framework. If nobody is developing RAC against iOS 7.x, it's not really supported anymore, is it?