Add missing dependency for documentation build
@@ -3,3 +3,8 @@ sphinx>=1.7
recommonmark==0.4.0
sphinx-copybutton
alabaster_jupyterhub
+traitlets
+# install BinderHub but because `pip install` is executed in the root
+# directory of this repository we write '.' here not '..' as you might
+# expect when thinking relative to this file.
+-e .
