Update kubernetes dependency to 1.15.0.

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

pkg/server/helpers.go

@@ -35,7 +35,7 @@ import (
 	"github.com/opencontainers/selinux/go-selinux/label"
 	"github.com/pkg/errors"
 	"golang.org/x/net/context"
-	runtime "k8s.io/kubernetes/pkg/kubelet/apis/cri/runtime/v1alpha2"
+	runtime "k8s.io/cri-api/pkg/apis/runtime/v1alpha2"
 
 	runtimeoptions "github.com/containerd/cri/pkg/api/runtimeoptions/v1"
 	criconfig "github.com/containerd/cri/pkg/config"