Update gen_rpc tag to 2.4.0

rebar.config

@@ -5,7 +5,7 @@
      {replayq, "0.1.1"}, %hex
      {esockd, "5.5.0"}, %hex
      {ekka, {git, "https://github.com/emqx/ekka", {tag, "v0.5.8"}}},
-     {gen_rpc, {git, "https://github.com/emqx/gen_rpc", {tag, "2.3.1"}}},
+     {gen_rpc, {git, "https://github.com/emqx/gen_rpc", {tag, "2.4.0"}}},
      {cuttlefish, {git, "https://github.com/emqx/cuttlefish", {tag, "v3.0.0"}}}
     ]}.