Use receipt["latest_receipt_info"] when verifying subscriptions

• See related PR: #134
• Should fix this: #129

Reference page:

https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Chapters/ValidateRemotely.html

Quoting:

latest_receipt_info: Only returned for iOS 6 style transaction receipts for auto-renewable subscriptions. The JSON representation of the receipt for the most recent renewal.

The values of the latest_receipt and latest_receipt_info keys are useful when checking whether an auto-renewable subscription is currently active. By providing any transaction receipt for the subscription and checking these values, you can get information about the currently-active subscription period. If the receipt being validated is for the latest renewal, the value for latest_receipt is the same as receipt-data (in the request) and the value for latest_receipt_info is the same as receipt.