Bundler binstubs makes rbenv rehash go away

if you use the binstubs feature of Bundler, then all your binaries will be local, and you shouldn't have to suffer the woes of forgetting to rehash things with rbenv (or installing that one plugin thing).

(note: with Rails 4, you'll need to be smarter about using binstubs, though -> https://twitter.com/the_chrismo/status/306557993493876737)