Modernize omnibus cookbook

- [x] Add ChefSpec coverage
- [x] Replace rbenv with chruby/ruby-install
- [x] Update to latest cookbook versions
- [x] Install bash
- [x] Release and bump build-essential dependency
- [ ] ~~Symlink everything in `/usr/local/bin` to `/usr/bin`~~ (rejected)
- [x] Add more ServerSpec

/cc @opscode-cookbooks/release-engineers