Use multiple buildpacks on your app
$ heroku config:add BUILDPACK_URL=https://github.com/ddollar/heroku-buildpack-multi.git
$ cat .buildpacks
https://github.com/heroku/heroku-buildpack-nodejs.git#versions
https://github.com/heroku/heroku-buildpack-ruby.git