Use a vendor directory for repeatable builds - fixes #816

This is using godep to manage the vendor directory.

vendor/google.golang.org/api/gensupport/doc.go

@@ -0,0 +1,10 @@
+// Copyright 2016 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 gensupport is an internal implementation detail used by code
+// generated by the google-api-go-generator tool.
+//
+// This package may be modified at any time without regard for backwards
+// compatibility. It should not be used directly by API users.
+package gensupport