Upgrade esockd and ekka deps

- Upgrade esockd to 5.6.1
- Upgrade ekka to 0.7.2

rebar.config

@@ -4,8 +4,8 @@
  [{gproc, "0.8.0"},
   {jiffy, "1.0.1"},
   {cowboy, {git, "https://github.com/emqx/cowboy", {tag, "2.7.1"}}},
-  {esockd, {git, "https://github.com/emqx/esockd", {tag, "5.6.0"}}},
-  {ekka, {git, "https://github.com/emqx/ekka", {tag, "0.7.1"}}},
+  {esockd, {git, "https://github.com/emqx/esockd", {tag, "5.6.1"}}},
+  {ekka, {git, "https://github.com/emqx/ekka", {tag, "0.7.2"}}},
   {gen_rpc, {git, "https://github.com/emqx/gen_rpc", {tag, "2.4.1"}}},
   {cuttlefish, {git, "https://github.com/emqx/cuttlefish", {tag, "v3.0.0"}}}
  ]}.