Update wp-social-login.php

Fix - Problem with login images protocol match with page protocol(https/http).

Error message from chrome console:
The page was loaded over HTTPS, but displayed insecure content from 'http://****/plugins/wordpress-social-login/assets/img/32x32/wpzoom/facebook.png': this content should also be loaded over HTTPS.

Info link: https://wordpress.org/support/topic/dont-use-wp_plugin_url
Merge pull request #42 from evgalak/master

Update wp-social-login.php