v13.7.0

Fix AppStore introctory prices

Fix a regression with introctory prices on iOS. Unclear when this happened,
according to Apple documentation, the "discounts" array should contain the
introctory prices, but it turns out it does not anymore.

Set ES6 as minimal javascript version

Down from ES2015, for broader compatibility.

Ensure verify() resolves even if there's no validator

Some user do not specify a receipt validator but want to call
"transaction.verify()" (for example app building frameworks).

This changes makes sure the behavior gets back like it used to be in earlier
versions of the plugin.