Droplet creation fails due to invalid image

[jchv]

It looks like the base image, "ubuntu-16-04-x64", is no longer available? When I try to do a deployment with nixops, it fails with the following error:

digitalocean.DataReadError: You specified an invalid image for Droplet creation.
exit status 1

This seems to trace back to here:

        self.manager = digitalocean.Manager(token=self.get_auth_token())
        droplet = digitalocean.Droplet(
            token=self.get_auth_token(),
            name=self.name,
            region=defn.region,
            ipv6=defn.enable_ipv6,
            ssh_keys=[ssh_key.public_key],
            image="ubuntu-16-04-x64",  # only for lustration
            size_slug=defn.size,
        )

        self.log_start("creating droplet ...")
        droplet.create()

It looks like someone attempted to update this in #23.

I think it's fair to say this project is not really maintained at the moment. Would it be reasonable to update the README to reflect this?