Update persistence for iOS 7

[mruegenberg]

When using iOS 7, there is a new process for persisting purchased non-consumable IAPs. See here:
"For non-consumable products and auto-renewable subscriptions in iOS 7 and later, use the app receipt as your persistent record."

[moleksyuk]

Hi @mruegenberg

Do you have any plans to fix this?

[mruegenberg]

Unfortunately, I don't have time at the moment, and it's not really a priority for me...

However, I do gladly accept pull requests!