Fix CAS login

Attempting to log in with CAS was giving a 500 error.

synapse/rest/client/v1/login.py

@@ -427,6 +427,7 @@ class CasTicketServlet(ClientV1RestServlet):
         self.cas_server_url = hs.config.cas_server_url
         self.cas_service_url = hs.config.cas_service_url
         self.cas_required_attributes = hs.config.cas_required_attributes
+        self.auth_handler = hs.get_auth_handler()
 
     @defer.inlineCallbacks
     def on_GET(self, request):