compile behaviour module first

rebar.config

@@ -18,6 +18,10 @@
 
 {validate_app_modules, true}.
 
+{erl_first_files, ["src/gen_server2.erl",
+                   "src/emqttd_auth_mod.erl",
+                   "src/emqttd_acl_mod.erl"]}.
+
 %% plugins cannot find emqttd.hrl without ".." lib dirs:(
 %% but this setting will make deps apps collision
 %% comment in 0.13.0 release