Handle the newer itunes receipt response

iTunes is sending the response for receipt validation differently now. It seems like older purchases still have the old format but this change will allow both responses to be properly parsed.

https://developer.apple.com/library/ios/releasenotes/General/ValidateAppStoreReceipt/Chapters/ReceiptFields.html#//apple_ref/doc/uid/TP40010573-CH106-SW1