npm http GET https://registry.npmjs.org/grunt-cli
npm http 200 https://registry.npmjs.org/grunt-cli
npm http GET https://registry.npmjs.org/grunt-cli/-/grunt-cli-0.1.9.tgz
npm http 200 https://registry.npmjs.org/grunt-cli/-/grunt-cli-0.1.9.tgz
npm http GET https://registry.npmjs.org/resolve
npm http GET https://registry.npmjs.org/findup-sync
npm http GET https://registry.npmjs.org/nopt
npm http 200 https://registry.npmjs.org/findup-sync
npm http GET https://registry.npmjs.org/findup-sync/-/findup-sync-0.1.2.tgz
npm http 200 https://registry.npmjs.org/nopt
npm http GET https://registry.npmjs.org/nopt/-/nopt-1.0.10.tgz
npm http 200 https://registry.npmjs.org/resolve
npm http GET https://registry.npmjs.org/resolve/-/resolve-0.3.1.tgz
npm http 200 https://registry.npmjs.org/findup-sync/-/findup-sync-0.1.2.tgz
npm http 200 https://registry.npmjs.org/nopt/-/nopt-1.0.10.tgz
npm http 200 https://registry.npmjs.org/resolve/-/resolve-0.3.1.tgz
npm http GET https://registry.npmjs.org/abbrev
npm http GET https://registry.npmjs.org/glob
npm http GET https://registry.npmjs.org/lodash
npm http 200 https://registry.npmjs.org/abbrev
npm http GET https://registry.npmjs.org/abbrev/-/abbrev-1.0.4.tgz
npm http 200 https://registry.npmjs.org/abbrev/-/abbrev-1.0.4.tgz
npm ERR! cb() never called!
npm ERR! not ok code 0
This only recently started happening. I spent about two hours trying to fix the issue (reinstalling grunt versions, etc), but in the end I just ran grunt manually and removed the buildpack, and it worked just fine.
Let me know if I can give you any more information.