Make client pipelines build dmgs.

Since `Omnibus.config[:sign_pkg] = false` is not working because omnibus is loading all the project files, temporarily switching over to having an additional file for client pipelines until we enable signing on chefdk slaves.
Update libyaml to 0.1.6 on windows.
Testing disk-cleanup branch of omnibus-ruby.
Merge pull request #142 from opscode/client-dmg

Enable dmg building in chef-client pipelines.