Update golang dependencies

vendor/google.golang.org/appengine/appengine_vm.go

@@ -1,20 +0,0 @@
-// Copyright 2015 Google Inc. All rights reserved.
-// Use of this source code is governed by the Apache 2.0
-// license that can be found in the LICENSE file.
-
-// +build !appengine
-
-package appengine
-
-import (
-	"golang.org/x/net/context"
-
-	"google.golang.org/appengine/internal"
-)
-
-// BackgroundContext returns a context not associated with a request.
-// This should only be used when not servicing a request.
-// This only works in App Engine "flexible environment".
-func BackgroundContext() context.Context {
-	return internal.BackgroundContext()
-}