Update vendors.

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

vendor/github.com/docker/docker/pkg/system/umask.go

@@ -0,0 +1,13 @@
+// +build !windows
+
+package system
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+// Umask sets current process's file mode creation mask to newmask
+// and returns oldmask.
+func Umask(newmask int) (oldmask int, err error) {
+	return unix.Umask(newmask), nil
+}