Some setup clarifications

[kjang96]

In docs/source/flow_setup.rst, some things are unclear:

• Let's take out push and popd. They don't do anything
• The instruction to visit the ray site to install dependencies should link directly to the dependency section, otherwise it appears to suggest the user should pip install ray, when in fact we just want them to install the dependencies.

[eugenevinitsky]

Resolved in #73