Add volatility dependency installation

[aarontp]

We have a task for volatility now (https://github.com/google/turbinia/pull/361), but volatility is not part of the dependencies. Need to look to see what version should be installed (pypi, debian, source, etc) and depending on which version it is, add a reference to it in either the setup.py extra_requires or the documentation.

[aarontp]

FYI @mwatkins-fb