Remove `vendor/bundle` if it already exists. Fixes #21.
this needs to happen before we install bundler or else we'll remove the bundler gem along with the user's gems
We need to do this to avoid issues with people adding their vendor/bundle directories in their git repo and causing issues with native extensions not being properly built. People should be using `bundle pack` instead. See heroku/heroku-buildpack-ruby#21.
also show time spent here