Update vendor to use Kubernetes 1.12.6

This is so that we can use more of `k8s.io/cli-runtime`

Gopkg.toml

@@ -19,21 +19,33 @@ required = [
   name = "github.com/spf13/cobra"
   branch = "master"
 
+[[override]]
+  name = "k8s.io/kubernetes"
+  version = "v1.12.6"
+
 [[override]]
   name = "k8s.io/api"
-  version = "kubernetes-1.11.3"
+  version = "kubernetes-1.12.6"
 
 [[override]]
   name = "k8s.io/apimachinery"
-  version = "kubernetes-1.11.3"
+  version = "kubernetes-1.12.6"
 
 [[override]]
   name = "k8s.io/client-go"
-  version = "kubernetes-1.11.3"
+  version = "kubernetes-1.12.6"
 
 [[constraint]]
-  branch = "master"
   name = "k8s.io/cli-runtime"
+  version = "kubernetes-1.12.6"
+
+[[override]]
+  name = "k8s.io/apiserver"
+  version = "kubernetes-1.12.6"
+
+[[override]]
+  name = "k8s.io/apiextensions-apiserver"
+  version = "kubernetes-1.12.6"
 
 [[constraint]]
   name = "sigs.k8s.io/yaml"
@@ -45,7 +57,7 @@ required = [
 
 [[constraint]]
   name = "k8s.io/kops"
-  branch = "master"
+  revision = "278e6606534eef97e9086563f030f6de25fb36ff"
 
 # copied from https://github.com/kubernetes/kops/blob/master/Gopkg.toml
 [[override]]