[close #277] Source default env from user
Merge pull request #279 from heroku/schneems/rack-env-overwrite-2

[close #277] Source default env from user
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.