Woocommerce Authorize.net CIM 3.10.9

* Fix - Ensure checkout can be completed after initial declines
* Fix - Ensure a payment method's last four are saved to the order when paying with a saved method

* Misc - Add support for WooCommerce 3.6
* Misc - Require PHP 5.4+

* Fix - Clear cached Client Keys when saving settings to prevent stale keys from being used
* Fix - Ensure the CSC field is passed to Accept.JS when enabled