Use Heroku buildpack cache possibility to store previously installed vendors

[Swop]

A cache folder is available during compilation time.

It would be nice to store in the cache the new downloaded vendor to be able to reuse them (if they didn't changed) in a future deploy, instead of download all of them form GitHub.

Heroku warns it's user that storing too much data in cache could slow the deployment process. But if the download of all vendor takes more time than the exploitation of cache data, maybe it's worth it.