Docker images are available at quay.io/jetstack/cert-manager-google-cas-issuer:0.6.2
One-line install: kubectl apply -f https://github.com/jetstack/google-cas-issuer/releases/download/v0.6.2/google-cas-issuer-v0.6.2.yaml
Includes a PR from @milesarmstrong - thanks for the contribution!
Thanks also to @JoshVanL, @SgtCoDFish, @wallrj and @inteon for their input!