* Important - Added support for WooCommerce 3.7+.
* Important - PHP 5.6.20+ is now required.
* Fix: Incorrect error message when 1 quantity is selected.
* Fix: Manually trigger product type change event in metabox on load.
* Tweak - Ignore 'woocommerce_before_cart_item_quantity_zero' action under WC 3.7+.