From SO answer
Your pip may be outdated. Even in Ubuntu 14.04 LTS, the pip version it installed using apt-get install python-pip was 1.5.4. Try updating pip manually, and possibly the new packages again as well.
pip --version # 1.5.4
curl -O https://bootstrap.pypa.io/get-pip.py
sudo python get-pip.py
hash -r # reset bash cache
pip --version # 6.0.8
The hash -r line is essential!
reference: https://pip.pypa.io/en/latest/installing.html