Enable SSL verification for urllib3

By default, urllib3 does not verify HTTPS requests.

As recommended in the urllib3 documentation [0], SSL verification is
enabled by including the certifi package which comes with Mozilla's
root certificate bundle. This silences an InsecureRequestWarning
otherwise issued by urllib3.

Before Python 2.7.9, the ssl module lacks SNI support and may lag
behind in security updates [1, 2, 3, 4]. The recommended approach by
urllib3 is to use pyOpenSSL as a replacement for those versions. This
silences InsecurePlatformWarning and SNIMissingWarning otherwise
issued by urllib3 when run on these versions.

[0]: https://urllib3.readthedocs.io/en/latest/user-guide.html#certificate-verification
[1]: https://urllib3.readthedocs.io/en/latest/user-guide.html#certificate-verification-in-python-2
[2]: http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-9365
[3]: https://www.python.org/dev/peps/pep-0476/
[4]: https://www.python.org/dev/peps/pep-0493/