Use bundel binstubs

When looking in to the rake tasks mentioned here https://github.com/StarkAndWayne/bosh-bootstrap/pull/73 I realized a bundle needed to be run before rake could be used. This already installed all the gems from source.

The only problem was these gems where not available in the path. So they had to be run with bundle exec from the root of the bosh repo. A solution for this is installing binstubs in a directory that is in the path. I have chosen /usr/bin but this could be change if this is not a best practice.