Update handler compatibility with tornado/pyzmq updates (#322)

Author: kevin-bates

kernel_gateway/jupyter_websocket/handlers.py

@@ -2,6 +2,7 @@
 # Distributed under the terms of the Modified BSD License.
 """Tornado handlers for kernel specs."""
 
+from notebook.utils import maybe_future
 from tornado import gen, web
 from ..mixins import TokenAuthorizationMixin, CORSMixin, JSONErrorsMixin
 import os
@@ -29,7 +30,7 @@ def get(self):
         resource_name, content_type = self.get_resource_metadata()
         self.set_header('Content-Type', content_type)
         res = web.StaticFileHandler.get(self, resource_name)
-        yield gen.maybe_future(res)
+        yield maybe_future(res)
 
     def options(self, **kwargs):
         """Method for properly handling CORS pre-flight"""