install sphinx dependencies

[kayvank]

Motiviation- make install fails with the error: exception: No module named 'sphinxcontrib.bibtex solution: install sphinxcontrib-bibtex dependency

[JoshOrndorff]

I was able to reproduce the error and confirm that the updated command resolves it on debian 9.

[dckc]

The dependency is in a requirements.txt file, so the traditional pip install -r requirements.txt might have been better, but whatever works.