Revert "cache_store can store files as well"

This reverts commit 4bfc6dcab6ecf3c015b7b0eab402a7c3a7adef93.

This would lead to successive slug size increases, storing the vendor
directory into the vendor/bundle directory each time. Like:

vendor/bundle/bundle/bundle/....
use `vendor/heroku` for storing metadata b/t builds
cleanup mess from v37
always write ruby_version to the cache
can't cache_load a file
reinstall language pack gems since they live in `vendor/bundle`
update changelog
don't clear cache on new apps
actually purge the cache again on ruby version change
always need to reinstall language gems when clearing the bundler cache
update changelog
store the buildpack version in the cache
this actually should just be an elsif
purge cache to fix bug in v38
refactor bundler_cache location
update changelog
update ruby build task for vulcan
fix typo in comments
Merge pull request #45 from schneems/schneems/typo

fix typo in comments
Update README.md

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