Update kubernetes dependency to 1.15.0.

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/server/update_runtime_config.go

@@ -25,7 +25,7 @@ import (
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/net/context"
-	runtime "k8s.io/kubernetes/pkg/kubelet/apis/cri/runtime/v1alpha2"
+	runtime "k8s.io/cri-api/pkg/apis/runtime/v1alpha2"
 )
 
 // cniConfigTemplate contains the values containerd will overwrite