ci: bump coveralls-erl -> v2.2.0-emqx-3

https://github.com/emqx/coveralls-erl/pull/4

rebar.config.erl

@@ -201,7 +201,7 @@ plugins() ->
 test_plugins() ->
     [
         {rebar3_proper, "0.12.1"},
-        {coveralls, {git, "https://github.com/emqx/coveralls-erl", {tag, "v2.2.0-emqx-2"}}}
+        {coveralls, {git, "https://github.com/emqx/coveralls-erl", {tag, "v2.2.0-emqx-3"}}}
     ].
 
 test_deps() ->