Woocommerce Software Add-on 1.9.2

* Fix - Change main plugin file
* Fix - Upgrade license form not working on simple products.
* Fix - Upgraded license cost not being applied correctly in cart.
* Tweak - Tested up to WooCommerce 3.3
* Update - WC tested up to WC 3.4
* Add - GDPR policy

* Fix - Add filter to requre user authentication (disabled by default, toggle filter `woocommerce_software_addon_api_require_user_auth`).

* New - Filter woocommerce_software_addon_save_license_key for altering licenses before they are saved.
* Fix - Lost license post call gives error when decoding JSON.
* Fix - Lost license woocommerce messages classes not correct.
* Fix - Purchasing multiple copies of software generates only a single
license.
* Fix - Generating new license keys on subscriptions renewals.