Update latest kubernetes version to v1.15.1
Update latest kubernetes version to v1.15.1

warmchang/minikube,kubernetes/minikube,dlorenc/minikube,dlorenc/minikube,dlorenc/minikube,warmchang/minikube,dlorenc/minikube,kubernetes/minikube,kubernetes/minikube,dalehamel/minikube,dalehamel/minikube,kubernetes/minikube,kubernetes/minikube,warmchang/minikube,warmchang/minikube,warmchang/minikube,warmchang/minikube,dlorenc/minikube,dlorenc/minikube,dalehamel/minikube,dlorenc/minikube,dlorenc/minikube,kubernetes/minikube,kubernetes/minikube,warmchang/minikube