Update ruby-nokogiri to 1.7.1.

# 1.7.1 / unreleased

## Security Notes

[MRI] Upstream libxml2 patches are applied to the vendored libxml 2.9.4 which address CVE-2016-4658 and CVE-2016-5131.

For more information:

* https://github.com/sparklemotion/nokogiri/issues/1615
* http://people.canonical.com/~ubuntu-security/cve/2016/CVE-2016-4658.html
* http://people.canonical.com/~ubuntu-security/cve/2016/CVE-2016-5131.html

## Dependencies

* [Windows] Upgrade zlib from 1.2.8 to 1.2.11 (unless --use-system-libraries)

(taca)