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