Update libyaml

Add libyaml 0.1.6.

This addresses the newly-released [CVE-2014-2525](https://www.ruby-lang.org/en/news/2014/03/29/heap-overflow-in-yaml-uri-escape-parsing-cve-2014-2525/) as well as [CVE-2013-6393](https://groups.google.com/forum/#!topic/rubysec-announce/3sx25iR7yHQ), which was addressed 0.1.5.