Upgrade dependencies

Author: gotthardp

rebar.config

@@ -18,7 +18,7 @@
     {worker_pool, {git, "https://github.com/inaka/worker_pool.git", {tag, "3.1.0"}}},
     {jsx, {git, "https://github.com/talentdeficit/jsx.git", {tag, "2.8.2"}}},
     {cowlib, {git, "https://github.com/ninenines/cowlib", {tag, "2.0.0-rc.1"}}},
-    {cowboy, {git, "https://github.com/ninenines/cowboy", {tag, "2.0.0-rc.3"}}},
+    {cowboy, {git, "https://github.com/ninenines/cowboy", {tag, "2.0.0-rc.4"}}},
     {gun, {git, "https://github.com/ninenines/gun.git", {tag, "1.0.0-pre.4"}}},
     {iso8601, {git, "https://github.com/erlsci/iso8601.git", {tag, "1.2.3"}}},
     {emqttc, {git, "https://github.com/emqtt/emqttc.git", {ref, "d4cbc38808fec0bda30be245268e876283c0e49f"}}}