In CI, lock the minikube version. This is important to ensure that when release branches are created from master, the minikube version is locked to a version that will forever support the desired k8s version. If the latest minikube version is used, CI for older releases will eventually break when the latest minikube no longer supports the older k8s version. Signed-off-by: Blaine Gardner <blaine.gardner@ibm.com>