WooCommerce Free Gift Coupons 3.7.0

* Fix: Undeprecate check_cart_items(). Should prevent multiple individual use coupons from being used simultaneously.

* IMPORTANT: WooCommerce 2.4 is minimum supported version
* New: WC 2.7 Compatibility - resolve 2.7 PHP warnings
* New: WC 2.7 Compatibility - use 2.7 CRUD when possible in admin metabox, and when saving order item meta
* New: New helper methods has_free_shipping() and get_gift_ids()
* Fix: Free gifts are removed from cart directly when coupon is removed, instead of via woocommerce_check_cart_items
* Fix: WC 2.7 Compatibility - remove previously added script, core automatically refreshes cart