change poison version for Phoenix 1.2 compatibility

mix.exs

@@ -29,7 +29,7 @@ defmodule Amplitude.Mixfile do
   defp deps do
     [
       {:httpoison, "~> 0.11.2"},
-      {:poison, "~> 3.1.0"}
+      {:poison, "~> 2.0"}
     ]
   end
 end