Include 'cowboy' as dependency in 'explorer/mix.exs'

The iex was not starting due to cowboy dependency error.

apps/explorer/mix.exs

@@ -97,7 +97,8 @@ defmodule Explorer.Mixfile do
       {:prometheus_ecto, "~> 1.3"},
       {:sobelow, ">= 0.7.0", only: [:dev, :test], runtime: false},
       {:timex, "~> 3.1.24"},
-      {:timex_ecto, "~> 3.2.1"}
+      {:timex_ecto, "~> 3.2.1"},
+      {:cowboy, "~> 1.0"}
     ]
   end