Upgrade to libnice stable release

[mqp]

Previously, we packaged a random git commit from libnice, because we really wanted bugfixes that were more recent than their stable release. However, nowadays, that's not the case -- libnice 0.1.15 or 0.1.16 should be quite satisfactory. We should upgrade when possible.

(I'm not just doing it right this instant because it will probably cause the usual cascading-dependencies-every-plan-needs-to-be-updated Habitat problems.)