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

jupyter_server/base/zmqhandlers.py

@@ -279,7 +279,7 @@ def get(self, *args, **kwargs):
         yield gen.maybe_future(res)
         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)
         self.session = Session(config=self.config)