Add Rails 4 binstubs to project

* This allows us to execute rake, cucumber and other registered binaries in
  the context of the bundle without the need for `bundle exec`
* To add new ones,
	`bundle binstubs some-gem-name`
  	`git add bin/new-executable`
Lose scripts/
Merge pull request #364 from alphagov/rails4-binstubs

Add Rails 4 binstubs to project