[CHEF-4901] Upgrade Chef Zero & move Puma back to development dependencies

https://tickets.opscode.com/browse/CHEF-4901

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.