Update binder to use conda, which allows us to install our own nodejs.

Our binder doesn’t work right now because the platform has node 10, but we now require node 12.

binder/environment.yml

@@ -0,0 +1,13 @@
+name: example-environment
+channels:
+  - conda-forge
+dependencies:
+  - jupyter-server-proxy
+  - matplotlib
+  - nodejs
+  - numpy
+  - pip
+  - python
+  - xeus-python
+  - pip:
+    - vega-datasets

binder/requirements.txt

@@ -1,4 +0,0 @@
-jupyter-server-proxy
-matplotlib
-vega-datasets
-xeus-python