Report the minimum required version of Ansible in the README.

Since we're using some cutting-edge Ansible stuff, you need to have a recent version of Ansible installed on your host machine. If you don't, you'll see some weird error messages halfway through provisioning (see #454).

This updates the README to document the minimum requirement, and tells you how to get an up-to-date version if you need to.

/cc @annegentle, @ycombinator