YITH WooCommerce Minimum Maximum Quantity Premium 1.39.0

* New: Support to WooCommerce 3.5.0 RC2
* Update: plugin framework
* Fix: cart validation

* Update: plugin framework
* Fix: cart validation

* New: integration with YITH WooCommerce Request a Quote Premium
* Update: Italian language
* Update: Spanish language
* Update: plugin framework
* Fix: checkout validation