Travis is not building the documentation index

Created by: Zaharid

The module index location does not exist:

https://docs.nnpdf.science/py-modindex.html

I think this is because validphys needs to be importable when we build the docs. Here https://travis-ci.com/NNPDF/nnpdf/jobs/237140764#L683 we are getting errors like No module named 'validphys'.

Should be trivial to fix installing the nnpdf package that was built in the previous step, here https://github.com/NNPDF/nnpdf/blob/master/.ciscripts/deploy-documentation.sh (good think we don't pay for bandwidth...).