use recommended iOS 7 version check

See: https://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSBundle_Class/Reference/Reference.html#//apple_ref/occ/instm/NSBundle/appStoreReceiptURL

"Note: The general best practice of weak linking using the respondsToSelector: method cannot be used here. Prior to iOS 7, the method was implemented as private SPI, but that implementation called the doesNotRecognizeSelector: method."