Switch to using the dep tool and update all the dependencies

vendor/golang.org/x/net/ipv6/sockopt_stub.go

@@ -0,0 +1,13 @@
+// Copyright 2013 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 nacl plan9
+
+package ipv6
+
+import "net"
+
+func getMTUInfo(s uintptr, opt *sockOpt) (*net.Interface, int, error) {
+	return nil, 0, errOpNoSupport
+}