Consider using/teaching binstubs

[rglassett]

https://github.com/sstephenson/rbenv/wiki/Understanding-binstubs

This could save keystrokes normally spent on bundle exec. There is a potential security risk in adding ./bin directly to PATH, but Thoughtbot has a clever workaround:

thoughtbot/dotfiles#215

Caveat: it does require running mkdir .git/safe once per trusted project. Would this be more or less confusing to students than simply running bundle exec [command]?