Update AuthenticationException.php

$_SERVER['SERVER_ADMIN'] might not always be set, especially if you are using nginx, instead of apache.

This produces a notice: 

PHP Notice: Undefined index: SERVER_ADMIN in \CAS\AuthenticationException.php on line 77

I added the following logic, 

if( isset($_SERVER['SERVER_ADMIN']) )
    return $_SERVER['SERVER_ADMIN'];
else
    return '';
Merge pull request #121 from arianf/patch-1

Update AuthenticationException.php