Update readme.md
Update readme.md

`npm test` failed with an error about template string. Template strings are not supported by latest version of node.js. So I assumed it requires io.js.
mathieujobin/heroku-docker,joeyespo/heroku-docker,mathieujobin/heroku-docker,matthew-andrews/heroku-docker,azylman/heroku-docker,heroku/heroku-docker,musicglue/heroku-docker,azylman/heroku-docker