WooCommerce Google Analytics Pro 2.2.0

* Tweak - Add tracking image to email additional content, instead of after the html closing tag

* Tweak - Send the user ID rather than the user login for signed in events
* Fix - Make sure the signed out event is shown under the My Account category rather than the general category
* Fix - Address a PHP warning that may be thrown in some circumstances when MonsterInsights is active
* Misc - Add support for WooCommerce 3.7
* Misc - Remove support for WooCommerce 2.6

* Misc - Add support for WooCommerce 3.6