CAS authentication for Wordpress

I have installed "WP Cassify" plugin to enable CAS login for my Wordpress.But after entering the username and password in CAS login screen, I am getting following error in my web browser:

Error [ wp_cassify_do_ssl_web_request ] : SSL certificate problem: self signed certificate

I don't know the reason behind this issue. I am using ubuntu 14.04.

via Chebli Mohamed