Update ekka tag

rebar.config

@@ -4,7 +4,7 @@
      {gproc, "0.8.0"}, % hex
      {replayq, "0.1.1"}, %hex
      {esockd, "5.5.0"}, %hex
-     {ekka, {git, "https://github.com/emqx/ekka", {branch, "master"}}},
+     {ekka, {git, "https://github.com/emqx/ekka", {tag, "v0.5.8"}}},
      {gen_rpc, {git, "https://github.com/emqx/gen_rpc", {tag, "2.3.1"}}},
      {cuttlefish, {git, "https://github.com/emqx/cuttlefish", {tag, "v3.0.0"}}}
     ]}.