Fix import issue

src/backend/backend.py

@@ -171,7 +171,7 @@ app = Application(urls, debug=ROUTER_DEBUG_MODE)
 
 def start_server(app, port):
     from tornado.ioloop import IOLoop
-    from tornado.options import enable_pretty_logging
+    from tornado.log import enable_pretty_logging
 
     enable_pretty_logging()