Fix doc builds

@wedaly @clytwynec 

This PR:

1. Breaks out the docs requirements so that readthedocs builds don't have to install selenium, browsermobproxy, psutils, and their subrequirements.
2. Updates the sphinx version to get the readthedocs builds working again.

You can see the successful readthedocs build here: https://readthedocs.org/builds/bok-choy/1742510/
And the doc here: http://bok-choy.readthedocs.org/en/test-docs/
I will remove the test-docs version from RTD after this PR gets merged.

Note that PDF builds are failing. I submitted this as Issue #69. 
We can look into that later, the problem I'm trying to solve right now is for the doc at readthedocs to be current.