Use conda_forge vega_datasets instead of pypi vega-datasets

Co-authored-by: Jeremy Tuloup <jeremy.tuloup@gmail.com>

binder/environment.yml

@@ -8,6 +8,5 @@ dependencies:
   - numpy
   - pip
   - python
+  - vega_datasets
   - xeus-python
-  - pip:
-    - vega-datasets