chore(deps): upgrade ekka to 0.7.4

rebar.config

@@ -7,7 +7,7 @@
   {jiffy, {git, "https://github.com/emqx/jiffy", {tag, "1.0.5"}}},
   {cowboy, {git, "https://github.com/emqx/cowboy", {tag, "2.7.1"}}},
   {esockd, {git, "https://github.com/emqx/esockd", {tag, "5.7.2"}}},
-  {ekka, {git, "https://github.com/emqx/ekka", {tag, "0.7.3"}}},
+  {ekka, {git, "https://github.com/emqx/ekka", {tag, "0.7.4"}}},
   {gen_rpc, {git, "https://github.com/emqx/gen_rpc", {tag, "2.4.1"}}},
   {cuttlefish, {git, "https://github.com/emqx/cuttlefish", {tag, "v3.0.0"}}}
  ]}.