I was doing this inside a virtualenv on Oracle Linux 6.4 using python-2.6 so the apt-based solutions weren't an option for me, nor were the python-2.7 ideas. My fix was to upgrade my version of setuptools that had been installed by virtualenv:
pip install --upgrade setuptools
After that, I was able to install packages into the virtualenv. I know this question has already had an answer selected but I hope this answer will help others in my situation.