The Jekyll Build Pack will look for a file named _config.yml in the app root and
run Jekyll to create and serve the site.
Add this language pack to your BUILDPACK_URL.
heroku config:add BUILDPACK_URL="https://github.com/jilion/heroku-buildpack-jekyll.git"
Or, with a recent version of the heroku gem, set it at creation time:
heroku create --stack cedar --buildpack https://github.com/jilion/heroku-buildpack-jekyll.git