README: Update kubernetes version to 1.18
Follow up from
@@ -10,7 +10,7 @@ not shipped with upstream Kubernetes.
## Features
-* Kubernetes 1.17 (upstream, via
+* Kubernetes 1.18 (upstream, via
[kubernetes-incubator/bootkube](https://github.com/kubernetes-incubator/bootkube))
* Fully self-hosted, including the kubelet
* Single or multi-master
