Drop explicit version from simple_zonal example

The explicit version in the simple_zonal example is causing failures as
1.11.4-gke.13 has been dropped as a valid master version. This commit
drops the explicit versions in the simple_zonal example; we can just use
the latest version avilable.

examples/simple_zonal/main.tf

@@ -34,8 +34,6 @@ module "gke" {
   subnetwork         = "${var.subnetwork}"
   ip_range_pods      = "${var.ip_range_pods}"
   ip_range_services  = "${var.ip_range_services}"
-  kubernetes_version = "1.11.4-gke.13"
-  node_version       = "1.11.4-gke.13"
   service_account    = "${var.compute_engine_service_account}"
 }