Update README.md with latest Release version
Update README.md with latest Release version

This is result of bug report #131

As far as I noticed version 1.3.1 doesn't support easy DigitalOcean auth with `doctl auth init`, but latest version 1.4.0 does.
I think it could be updated to make people install latest version with all updates.
digitalocean/doctl,digitalocean/doctl,xmudrii/doctl,xmudrii/doctl