Upgrade to Omnibus 4.0

Experience tomorrow's future...today. _Act now_ and Releng will throw in one new test platform for __free__.

- [x] Full CI test run: http://wilson.ci.opscode.us/job/chef-server-12-trigger-ad_hoc/204/downstreambuildview/

/cc @opscode/release-engineers @opscode/server-team 

Because `omnibus.rb` is checked in the repo now. Check the commit message :)
https://github.com/opscode/opscode-omnibus/commit/0d26caebaee4fd6478c5a6b20a530969d4eff74b