WooCommerce Social Login 2.15.1

* Tweak - Add support for LinkedIn API v2
* Fix - Handle properly OAuth requests errors
* Fix - Add missing options key for WPML translation
* Fix - Do not display admin notices for lack of SSL support on newer WooCommerce versions running on HTTPS
* Dev - Filter the callback URL format
* Misc - Upgrade HybridAuth authentication library to v2.14.0

* Misc - Add support for WooCommerce 3.6
* Misc - Require PHP 5.4+

* Fix - As PayPal may not allow sharing users email addresses without approval, do not require email for new installations and let users that sign in with PayPal provide one manually instead
* Dev - Add filter hook to alter the request scope of PayPal profile fields and add more fields
* Dev - Remove SkyVerge Framework methods from template files for better backwards compatibility