Allow CAS_CLIENT_SERVICE to be given as comma separated string

The phpCAS methods allow an array of URLs to be given for the service base. The previous implementation unnecessarily limited this package to a single string value. The package config file will now explode the value on commas, resulting in an array of possible values.

src/config/config.php

@@ -65,7 +65,7 @@
     |--------------------------------------------------------------------------
     | Example: 'http://localhost', 'https://example.com:8888'
     */
-    'cas_client_service' => env('CAS_CLIENT_SERVICE', 'http://localhost'),
+    'cas_client_service' => explode(',', env('CAS_CLIENT_SERVICE', 'http://localhost')),
 
 
     /*