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.

Here, i think we can skip loading the cache if we're going to purge it? Might save a few milliseconds not having to copy files on disk just to delete them again. Could put this in an `else` in the branching logic below.