p2p/discover: ignore packet version numbers

The strict matching can get in the way of protocol upgrades.

p2p/discover/udp.go

@@ -39,7 +39,6 @@ var (
 	errPacketTooSmall   = errors.New("too small")
 	errBadHash          = errors.New("bad hash")
 	errExpired          = errors.New("expired")
-	errBadVersion       = errors.New("version mismatch")
 	errUnsolicitedReply = errors.New("unsolicited reply")
 	errUnknownNode      = errors.New("unknown node")
 	errTimeout          = errors.New("RPC timeout")
@@ -522,9 +521,6 @@ func (req *ping) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) er
 	if expired(req.Expiration) {
 		return errExpired
 	}
-	if req.Version != Version {
-		return errBadVersion
-	}
 	t.send(from, pongPacket, pong{
 		To:         makeEndpoint(from, req.From.TCP),
 		ReplyTok:   mac,