Fixing GCR Client for GKE images which used to be hosted under `google-containers`

[davidcollom]

This is related to https://github.com/jetstack/version-checker/issues/147

Actually https://gcr.io/v2/google-containers/kube-proxy-amd64/tags/list hasn't been updated for a long time because they switched to https://gke.gcr.io/v2/kube-proxy-amd64/tags/list and new images are published there.

Google has now stopped pushing to google-containers namespace and now all gcr URI's can have multiple nested paths or none what so ever under their own vanity domain (gke.gcr.io)