MailChimp for WooCommerce Memberships 1.5.0

* Fix - Ensure the Debug Mode setting can be disabled

* Fix - Address a PHP callback error when using WooCommerce versions earlier than 3.7

* Misc - Add support for WooCommerce 3.7
* Misc - Remove support for WooCommerce 2.6
* Misc - Require PHP 5.6+