refactor(emqx_coap): pin gen_coap 0.3.1 for dialyzer warning fix

apps/emqx_coap/rebar.config

@@ -1,6 +1,6 @@
 {deps,
  [
-  {gen_coap, {git, "https://github.com/emqx/gen_coap", {tag, "v0.3.0"}}}
+  {gen_coap, {git, "https://github.com/emqx/gen_coap", {tag, "v0.3.1"}}}
  ]}.
 
 {edoc_opts, [{preprocess, true}]}.