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 requests library that happens to be installed in the system python.
This adds the client's requirements explicitly to the docs_requirements.txt
so that this broken version of requests is ignored.