Don't pass spawn=None when creating WSGIServer.

New versions of gevent seem unhappy about this (raise exception with message
"Impossible to call blocking function in the event loop callback" when
handling an incoming HTTP request). I don't quite understand what that
parameter is supposed to do and I think I don't it.

check_xmpp_dns.py

@@ -412,4 +412,4 @@ def application(env, start_response):
 
 if __name__ == '__main__':
     logging.basicConfig(filename='log')
-    gevent.wsgi.WSGIServer(('', 1000), application=application, spawn=None).serve_forever()
+    gevent.wsgi.WSGIServer(('', 1000), application=application).serve_forever()