vendor: update golang.org/x/oauth2 and cloud.google.com/go

Pick up the bug fix for golang/oauth2#237
Bump up cloud.google.com/go as a result of updating the OAuth2 code.

This commit was generated by:
./hack/pin-dependency.sh golang.org/x/oauth2 9f3314589c9a
./hack/pin-dependency.sh cloud.google.com/go v0.34.0
./hack/update-vendor.sh
git add vendor
git add -u

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

@@ -0,0 +1,6 @@
+// Copyright 2017 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.
+
+// Package internal contains support packages for oauth2 package.
+package internal