add nteract_on_jupyter via pip in conda

repo2docker/buildpacks/conda/environment.yml

@@ -1,5 +1,7 @@
 dependencies:
-  - python==3.6.*
+  - python=3.6.*
   - ipywidgets==6.0.1
   - jupyterlab==0.30.6
   - notebook==5.2.2
+  - pip:
+    - nteract_on_jupyter==1.3.1

repo2docker/buildpacks/conda/install-miniconda.bash

@@ -31,6 +31,8 @@ echo 'update_dependencies: false' >> ${CONDA_DIR}/.condarc
 ${CONDA_DIR}/bin/conda config --system --set show_channel_urls true
 
 ${CONDA_DIR}/bin/conda env update -n root -f /tmp/environment.yml
+# enable nteract-on-jupyter, which was installed with pip
+jupyter serverextension enable nteract_on_jupyter --sys-prefix
 
 if [[ -f /tmp/kernel-environment.yml ]]; then
     # install kernel env and register kernelspec