WooCommerce Google Analytics Pro 2.2.0

* Fix - Ensure custom translation files are correctly loaded when the plugin initializes
* Misc - Replace the Google API client library with a lightweight internal API handler
* Misc - Remove Guzzle dependency
* Localization - Add Italian translation

* Misc - Update Guzzle to v6
* Misc - Require PHP 5.6+

* Tweak - Send the document host name parameter in measurement protocol API requests
* Fix - Harden code that may have produced a PHP error when a product can't be determined for ecommerce data JavaScript output
* Fix - Prevent a PHP error from happening while refreshing the access token in case of remote server errors