Use vendoring for backward compatibility
The switch of the Madon library to Go modules breaks builds with
Go version < 1.11 since the import path now contains "v2".
This patch adds vendoring so that it can still build with those
versions.
Let's try to re-enable Travis builds with Go v1.8-1.11...
// 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.
//
// +build 386 amd64 amd64p32 arm arm64 ppc64le mipsle mips64le
package unix
const isBigEndian = false