chore: update emqtt vsn

apps/emqx/rebar.config

@@ -43,7 +43,7 @@
             {meck, "0.9.2"},
             {proper, "1.4.0"},
             {bbmustache, "1.10.0"},
-            {emqtt, {git, "https://github.com/emqx/emqtt", {tag, "1.6.0"}}}
+            {emqtt, {git, "https://github.com/emqx/emqtt", {tag, "1.7.0-rc.1"}}}
         ]},
         {extra_src_dirs, [{"test", [recursive]}]}
     ]}