Merge pull request #275 from heroku/schneems/bundler-1.6.3

bundler 1.6.3
bump to v120
[ci skip] v120 changelog
[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
bundler cache is now stack aware

vendor/bundle -> stack/vendor/bundle