Forgot default here

Author: rcthomas

jupyter_server_proxy/config.py

@@ -2,7 +2,7 @@
 Traitlets based configuration for jupyter_server_proxy
 """
 from notebook.utils import url_path_join as ujoin
-from traitlets import Dict, List, Union
+from traitlets import Dict, List, Union, default
 from traitlets.config import Configurable
 from .handlers import SuperviseAndProxyHandler, AddSlashHandler
 import pkg_resources