WooCommerce Social Login 2.16.0

* Fix - Replace deprecated `is_ajax()` with `wp_doing_ajax()`
* Misc - Require WooCommerce 3.9.4 or newer

* Localization - Add Russian translation (thanks to Paley)

* Misc - Add compatibility for WooCommerce 4.7
* Misc - Require PHP 7.0 or newer