update plug_cowboy to 2.7

Author: zaid

mix.exs

@@ -52,7 +52,7 @@ defmodule EvlDaemon.Mixfile do
       {:httpoison, "~> 1.7.0"},
       {:jason, "~> 1.2"},
       {:plug, "~> 1.15"},
-      {:plug_cowboy, "~> 2.6"},
+      {:plug_cowboy, "~> 2.7"},
       {:cors_plug, "~> 2.0"},
       {:dialyxir, "~> 1.1.0", only: [:dev], runtime: false},
       {:ex_doc, "~> 0.22.2", only: [:dev], runtime: false},