Call tornado WebSocketHandler.get() as a coroutine

This change will probably be needed for Tornado 6. Ben Darnell figured it out.
https://groups.google.com/d/msg/python-tornado/mSeG7Kc6z4o/baeTDOvJGgAJ

Author: takluyver

notebook/base/zmqhandlers.py

@@ -281,7 +281,8 @@ def get(self, *args, **kwargs):
         # assign and yield in two step to avoid tornado 3 issues
         res = self.pre_get()
         yield gen.maybe_future(res)
-        super(AuthenticatedZMQStreamHandler, self).get(*args, **kwargs)
+        res = super(AuthenticatedZMQStreamHandler, self).get(*args, **kwargs)
+        yield gen.maybe_future(res)
 
     def initialize(self):
         self.log.debug("Initializing websocket connection %s", self.request.path)