third-party: update libyaml submodule

Need to update tests as with fixup in upstrem
commit baf636a74b4b6d055d93e2d01366d6097eb82d90
Author: Tina Müller <cpan2@tinita.de>
Date:   Thu Jun 14 19:27:04 2018 +0200

The closing single quote needs to be indented...
if it's on its own line.

Closes #3275.