fix(readthedocs): install CLI requirements explicitly

Readthedocs.org began failing to generate our Python code autodocs
recently. Purging the build environment (use https://readthedocs.org/wipe/deis/latest/)
did not help. Their logs indicate Sphinx is confused by a version of
the urllib3 library that happens to be installed in the system python.
This packaging error was fixed in urllib3 1.8.2, see https://pypi.python.org/pypi/urllib3.
This adds the client's requirements explicitly to the docs_requirements.txt
so that this broken version of urllib3 isn't used.

This is also fixed by requests 2.3.0, but pinning urllib3 seemed like
a safer fix.