Give - GoCardless Gateway 1.3.9

* New: Added support for new recurring options found in Recurring Donations 1.6+ - please update recurring if you're using it alongside GoCardless!
* Fix: Incorrect link to GoCardless webhook secret page.
* Fix: Validate "day" billing period for GoCardless - this gateway does not support daily recurring donations so we've built in validation to prevent admins from setting daily recurring donations.

* Fix: PHP warning causing AJAX issues with certain servers and versions of GiveWP.

* Fix: Recurring subscriptions would not display the proper subscription text within the GoCardless page. This has been resolved so now all forms of recurring now display properly at the GoCardless checkout page.