[libyaml] mercurial repos gone; use github.

[truthbk]

No description provided.

[olivielpeau]

I've done some more digging:

• it looks like the commit pinned on the mercurial repo was added in this PR: [libyaml] Pin libyaml version to the current tip #59, previously we were pulling the tip of master. This was merged on Jun 21, 2016
• 0.1.6 was tagged in 2014, 2 years prior: https://github.com/yaml/libyaml/releases/tag/0.1.6, whereas 0.1.7 was tagged on Aug 27, 2016: https://github.com/yaml/libyaml/releases/tag/0.1.7. In particular this fix was included in both 0.1.7 and (based on dates) in the version we were previously pinning, looks important: yaml/libyaml@686b2d8
• we pin 0.1.7 in the windows software def: https://github.com/DataDog/omnibus-software/blob/agent-v5/config/software/libyaml-windows.rb (haven't checked we're actually using it for agent 5 though :) )

(we could go as far as pinning the same version as v6/v7 agents, i.e. 0.2.2 -- see #326 -- since we've upgraded pyyaml in Agent v5 as well; but there seems to be no CVEs since 0.1.7, so not needed IMO, see https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=libyaml)

Anyway, overall I think pinning 0.1.7 is best.