mkdir /root/bin

This fixes some issues I've seen around the web with php buildpack.  See:

* https://github.com/drmikecrowe/dokku-buildpack-php/commit/00cf93c2f3a60e11f9d4e81a7c110977dbddee41
* https://github.com/CHH/heroku-buildpack-php/issues/160
* https://github.com/romaninsh/dokku-alt-manager/issues/5