Downgrade to berkshelf stable, since 3.0.0 beta is problematic

Berkshelf is in the process of moving from Riot Games to Opscode maintainership.  In the meantime, http://api.berkshelf.com is down and the beta version is proving a liability.  Once the transition stabilizes, we can switch back to Berkshelf 3.

It would appear we lose support for `--ssl-verify=false` (non-SSL Chef Server?) -- but we need the stability right now.