Update libyaml to version 0.1.6

This is required to close the following security problem:

http://www.ocert.org/advisories/ocert-2014-003.html
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-2525

I've run the bats tests, and got full marks on them. Please let me know if I can do anything to help expedite this PR or the subsequent release!