fix(ekka): Upgrade to 0.7.6 to allow remsh

Before ekka 0.7.6, when epmd is not used, ekka_dist would calculate
a port number from node name. This does not work for remote consoles
because a remsh node name is like remsh81random0-<target-node>
i.e. the calculated port number is always the same
as the target node.

The fix in 0.7.6 allows remsh prefixed nodes to use a range of ports
to listen.

rebar.config

@@ -7,7 +7,7 @@
   {jiffy, {git, "https://github.com/emqx/jiffy", {tag, "1.0.5"}}},
   {cowboy, {git, "https://github.com/emqx/cowboy", {tag, "2.7.1"}}},
   {esockd, {git, "https://github.com/emqx/esockd", {tag, "5.7.4"}}},
-  {ekka, {git, "https://github.com/emqx/ekka", {tag, "0.7.5"}}},
+  {ekka, {git, "https://github.com/emqx/ekka", {tag, "0.7.6"}}},
   {gen_rpc, {git, "https://github.com/emqx/gen_rpc", {tag, "2.5.0"}}},
   {cuttlefish, {git, "https://github.com/emqx/cuttlefish", {tag, "v3.0.0"}}}
  ]}.