support STACK for different cedar stacks
prevent directory traversal
Merge pull request #282 from heroku/cedar-14

support STACK for different cedar stacks
update changelog
bump to v121
add cache documentation
make warnings more obvious
Merge pull request #285 from heroku/schneems/warnings

make warnings more obvious
unfocus test
bump hatchet
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
update changelog
bump to v122
rev hatchet to fix parallel app delete problem
Merge pull request #291 from heroku/rev-hatchet

rev hatchet to fix parallel app delete problem
need to clear the destination bundle store

for instance in minitest:
-r--r--r-- 1 hone hone  2003 Jul 25 16:33 Rakefile

Don't have write permission, so the buildpack pukes when trying to
overwrite the cache.
don't load the cache if we just converted the stack

the cache will already be loaded
Merge pull request #293 from hone/debug-cedar-14

fix cedar-14 bundler cache bugs