use go 1.12

Signed-off-by: hongming <talonwan@yunify.com>

vendor/golang.org/x/oauth2/internal/client_appengine.go

@@ -1,13 +0,0 @@
-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build appengine
-
-package internal
-
-import "google.golang.org/appengine/urlfetch"
-
-func init() {
-	appengineClientHook = urlfetch.Client
-}