Rename RMAppReceipt's hash to receiptHash to not clash with NSObject.hash

This fixes building against iOS 8.0 beta 4 after changes to NSObject.h:
https://developer.apple.com/library/prerelease/ios/releasenotes/General/iOS80APIDiffs/index.html
Merge pull request #91 from kommen/ios8-hash

Rename RMAppReceipt's hash to receiptHash to not clash with NSObject.hash