[WIP] Point at Omnibus `schisamo/appbundle-dsl` branch
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`