SSL certificate verification workaround.

Python 2.7.9 performs SSL certificate verification by default (see [issue #22417](https://hg.python.org/cpython/raw-file/v2.7.9/Misc/NEWS) and [PEP 476](https://www.python.org/dev/peps/pep-0476/)). On Travis-CI (and on my local, RHEL 6 machine) the verification is failing for GitHub when attempting to download Natural Earth shapefiles.

This PR is intended to be a *temporary* workaround which rolls Python back to 2.7.8 where the certificates are unchecked.