YITH WooCommerce Color and Label Variations Premium 2.8.1

* New: Support to WooCommerce 3.5.0
* Update: Plugin Core.
* Update Languages files.
* Fix: Error can't use function return value in write context.
* Fix: Image size for variations image in archive pages.

* Fix: JavaScript error for custom product attributes.

* New: Support to WooCommerce 3.4.5.
* New: Support to WordPress 4.9.8.
* New: Add different images gallery per variation.
* Update: Spanish language
* Update: Plugin Core.
* Fix: Compatibility with Flatsome Infinite Scrolling
* Dev: Refactoring plugin frontend javascript.