Create a conda environment (or virtual environment) using environment.yml.
Activate the environment:
$ source activate [NameOfEnv]
Enable Jupyter notebook wihtin the environment:
$ conda install ipykernel --name [NameOfEnv]
$ python -m ipykernel install
Install npm.
Then, enable widgets and leaflet in in Jupyter notebooks:
$ jupyter nbextension install --py --symlink --sys-prefix widgetsnbextension
$ jupyter nbextension enable --py --sys-prefix widgetsnbextension
$ jupyter nbextension install --py --symlink --sys-prefix ipyleaflet
$ jupyter nbextension enable --py --sys-prefix ipyleaflet