WooCommerce Sequential Order Numbers Pro 1.21.4

* Fix - When filtering results by order number in WooCommerce Admin Downloads Analytics, search should handle sequential order numbers properly

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

* Fix - Return correct order numbers in WooCommerce REST API responses for v3 and v2
* Misc - Drop support for PHP v5.2: PHP v5.3 is now the minimum supported version
* Misc - Update SkyVerge plugin framework to version 5.3
* Misc - Change the plugin main file to woocommerce-sequential-order-numbers-pro.php to match container directory