This is a build pack for Middleman that will create your static site.
It is cleaner than most Middleman build packs out there because it takes advantage of the heroku-buildpack-multi buildpack to separate out the Ruby and Middleman specific components.
This build pack is meant to be used with the heroku-buildpack-multi buildpack. Setup generally goes like this with an existing Heroku app:
$ heroku config:add BUILDPACK_URL=https://github.com/ddollar/heroku-buildpack-multi.git
...
$ cat .buildpacks
https://github.com/heroku/heroku-buildpack-ruby.git
https://github.com/hashicorp/heroku-buildpack-middleman.git
Then just push!