WooCommerce Customer Order CSV Export 5.5.3

* Fix - Fix a bug which caused exports not to work without re-saving settings after install
* Misc - Add support for WooCommerce 3.7
* Misc - Remove support for WooCommerce 2.6
* Misc - Require PHP 5.6+

* Fix - Only test loopback connection on plugin settings page

* Feature - Create and manage multiple custom formats for order, customer, and coupon exports
* Misc - Update the SkyVerge Plugin Framework to version 5