WooCommerce Social Login 2.15.1

* Misc - Add a warning that the Instagram integration will be retired in early 2020
* Misc - The Instagram integration is no longer available for new installs

* Misc - Add support for WooCommerce 3.8

* Fix - Fix text domain and MC ref param in setup wizard
* Fix - Ensure VK provider is able to work with VK API v5
* Misc - Update HybridAuth to version 2.16