Updated Omnibus with `appbundle` DSL
Use Omnibus’s new `appbundle` method in Builder DSL
Perform `rubygems-customization` after all gems are installed

This ensures bundle installs that are executed during the Omnibus build
don’t end up in the build user’s home directory:

  `~/.chefdk/gem/ruby/2.1.0/gems/`
Bin files from non-Appbundled gems belong in `embedded/bin`
Remove crazy PATH on Chef Windows gem `bundle install`

`with_embedded_path` does the right thing now.
No need to delete the Windows gem on Chef install

Since we create the gems with targeted `gem build` commands there is
nothing to delete any longer!