Make ruby buildpack a requirement for this nanoc buildpack.

Ruby buildpack makes a better job processing your Gemfile. For example,
it can install custom ruby version.

To deploy nanoc site with this buildpack you need:
- use heroku multi buildpack
  https://github.com/ddollar/heroku-buildpack-multi
- configure multi buildpack to use both ruby and nanoc buildpacks:
  .buildpacks:
    git://github.com/heroku/heroku-buildpack-ruby.git
    git://github.com/eligo/heroku-buildpack-nanoc.git