phpCAS 1.3.3 migration due to security issues

Hi Patrick,

It seems the phpCAS version 1.1.3, actually used by the plugin, isn't supported anymore and has many security issues (See https://wiki.jasig.org/display/casc/phpcas).

I've upgraded your plugin to use the latest version of the phpCAS library (1.3.3). I didn't have the time to check every functionalities but I can assure that basic authentication (Without proxy or certificate) is working on login/logout. Cron is working but I haven't checked the sync.

Hope it helps to anybody who's currently using this plugin.

Jean-Philippe