heroku/heroku-buildpack-nodejs — pull request #115 by kuba-kubula (2014-07-04 09:18:18+00:00)
Title: Prune any past devDependencies, if present in cache

If the --production flag is specified, this command will remove the packages specified in your devDependencies.

https://www.npmjs.org/doc/cli/npm-prune.html