bundler cache is now stack aware

make the bundler cache stack aware. This means in the cache directory:
`vendor/bundle` -> `stack/vendor/bundle`. So cedar will be
`cedar/vendor/bundle`. The layout on the slug itself will be unchanged.

This is how the migration will work.
* we assume you're coming from the cedar stack, so it won't blow the
* cache away if have nothing set.
* going from cedar to cedar-14 will clear the cache or vice versa
* having no stack known to the cache to cedar-14 will clear the cache
* after this, everyone will have a stack aware bundler cache
* the layout on the slug itself will be unchanged. only the cache
* directory will take the stack into account.
add stack change spec tests
Merge pull request #286 from heroku/cedar-14

bundler cache is now stack aware