Fix docker-py requirement in setup.py

I updated requirements.txt but forgot about setup.py, so the version on
pypi is broken for people who already have docker-py 0.5.0 installed.

Closes #547.

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>
Merge pull request #548 from aanand/fix-docker-py-requirement

Fix docker-py requirement in setup.py