[go.mod] upgrade libp2p-pubsub to 0.3.1

Signed-off-by: Leo Chen <leo@harmony.one>

go.mod

@@ -44,7 +44,7 @@ require (
 	github.com/libp2p/go-libp2p-net v0.1.0
 	github.com/libp2p/go-libp2p-peer v0.2.0
 	github.com/libp2p/go-libp2p-peerstore v0.2.4
-	github.com/libp2p/go-libp2p-pubsub v0.3.0
+	github.com/libp2p/go-libp2p-pubsub v0.3.1
 	github.com/multiformats/go-multiaddr v0.2.2
 	github.com/multiformats/go-multiaddr-net v0.1.5
 	github.com/natefinch/lumberjack v2.0.0+incompatible