pip >= 1.5 needs --allow-all-external

Starting from version 1.5, pip stopped downloading packages hosted in an
external site, even if there's a checksum in PyPI that we can check the
resulting package against. This breaks when downloading argparse==1.2.1 with
Python 2.6.

Here we allow all external downloads. It shouldn't be a security issue, as
we're still checking the package's hash against PyPI.

For more info, check
http://stackoverflow.com/questions/21021326/security-considerations-of-pip-allow-external