Fix terminals with Tornado 3

The websocket handler auth checking was calling clear_cookie(), which
threw an error because it doesn't make sense for Websockets. It doesn't
seem important, and we silence it in our other websocket handlers, so
silencing it here too.

IPython/html/terminal/handlers.py

@@ -33,6 +33,10 b' class TermSocket(terminado.TermSocket, IPythonHandler):'
         if tornado.version_info >= (4,):
             return super(TermSocket, self).get(*args, **kwargs)
 
+    def clear_cookie(self, *args, **kwargs):
+        """meaningless for websockets"""
+        pass
+
     def open(self, *args, **kwargs):
         if tornado.version_info < (4,):
             try: