Use a vendor directory for repeatable builds - fixes #816

This is using godep to manage the vendor directory.

vendor/golang.org/x/sys/unix/asm.s

@@ -0,0 +1,10 @@
+// Copyright 2014 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 !gccgo
+
+#include "textflag.h"
+
+TEXT ·use(SB),NOSPLIT,$0
+	RET