Ensure `GitFetcher` properly resolves remote refs

This fixes a regression introduced in: 2f240e4
Merge pull request #341 from opscode/schisamo/git-remote-ref-fix

Ensure `GitFetcher` properly resolves remote refs
Add an `appbundle` function to the builder DSL

This function takes advantage of an embedded copy of `appbundler`.
Appbundler reads a project’s Gemfile.lock and generates locked binstubs.