Only include VOEvent 2.0 servers in default hosts

This way, we eliminate the surprise that we might cause the user
by sudden changes in VOEvent schema.

gcn/voeventclient.py

@@ -217,8 +217,7 @@ def _validate_host_port(host, port):
     return host, port
 
 
-def listen(host=("209.208.78.170", "45.58.43.186", "50.116.49.68",
-           "68.169.57.253"), port=8099,
+def listen(host=("45.58.43.186", "68.169.57.253"), port=8099,
            ivorn="ivo://python_voeventclient/anonymous", iamalive_timeout=150,
            max_reconnect_timeout=1024, handler=None, log=None):
     """Connect to a VOEvent Transport Protocol server on the given `host` and