Upgrade Chef Zero & remove Puma

This commit upgrades Chef Zero to version 2.0.0, which removed Puma entirely.
Chef no longer requires Puma at runtime, so it can be moved into the
development gem group. Chef Zero 2.0 runs entirely in WEBrick and also uses
less memory and a more direct threading approach.
Changelog for chef-zero upgrade
Merge branch 'CHEF-4901'