chore(ekka): bump ekka to 0.8.1.6

We're doing this to remove some missing change warnings when updating
emqx v4.3.10.  Ekka's appup was updated in emqx/ekka#140 .

rebar.config

@@ -43,7 +43,7 @@
     , {jiffy, {git, "https://github.com/emqx/jiffy", {tag, "1.0.5"}}}
     , {cowboy, {git, "https://github.com/emqx/cowboy", {tag, "2.8.2"}}}
     , {esockd, {git, "https://github.com/emqx/esockd", {tag, "5.8.0"}}}
-    , {ekka, {git, "https://github.com/emqx/ekka", {tag, "0.8.1.5"}}}
+    , {ekka, {git, "https://github.com/emqx/ekka", {tag, "0.8.1.6"}}}
     , {gen_rpc, {git, "https://github.com/emqx/gen_rpc", {tag, "2.5.1"}}}
     , {cuttlefish, {git, "https://github.com/emqx/cuttlefish", {tag, "v3.3.6"}}}
     , {minirest, {git, "https://github.com/emqx/minirest", {tag, "0.3.7"}}}