missing dependency in conda environment

[brian-rose]

I tried to build the portal site locally. From the repository root:

conda env create -f ci/environment.yml
conda activate
make html

generated the following error:

Extension error:
Could not import extension gallery_generator (exception: No module named 'pandas')
make: *** [html] Error 2

This was quickly fixed by doing conda install pandas.

I will submit a PR to add pandas to environment.yml