fix go.sum (iotexproject#2412)

Author: dustinxie

go.mod

@@ -10,6 +10,7 @@ require (
 	github.com/golang/groupcache v0.0.0-20191027212112-611e8accdfc9
 	github.com/golang/mock v1.4.0
 	github.com/golang/protobuf v1.4.2
+	github.com/gorilla/websocket v1.4.0
 	github.com/grpc-ecosystem/go-grpc-middleware v1.2.0
 	github.com/grpc-ecosystem/go-grpc-prometheus v1.2.0
 	github.com/hashicorp/golang-lru v0.5.1