Poison.decode/2 is undefined in production

Question

My function in elixir 1.9 decodes json response from an external api and looks like this:

def decode({ok, body, "application/json"}) when is_binary(body) do
    body
    |> Poison.decode(keys: :atoms)
    |> case do
       {:ok, parsed} -> {ok, parsed}
       _ -> {:error, body}
     end 
end

It works very well in development but crashes the application with the following error message:

 ** (EXIT) an exception was raised:
    ** (UndefinedFunctionError) function Poison.decode/2 is undefined (module Poison is not available)

Here's my mix.ex where dependencies have been included:

defp deps do
 [
  # {:dep_from_hexpm, "~> 0.3.0"},
  # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
  { :credo, "~> 0.4", only: [:dev, :test] },
  { :httpoison, "~> 1.6" },
  { :plug_cowboy, "~> 2.0" },
  { :poison, "~> 3.1" },
  { :plug_logger_json, "~> 0.7.0" },
  { :stash, "~> 1.0.0" },
  {:edeliver, ">= 1.6.0"},
  {:distillery, "~> 2.0", warn_missing: false},
  ]
end

And this is how I compiled my production build:

 MIX_ENV=prod mix deps.clean --all 
 && MIX_ENV=prod mix deps.get 
 && MIX_ENV=prod mix clean 
 && MIX_ENV=prod mix compile 
 && MIX_ENV=prod mix release my_app

mix.exs application/0

  def application do
    [
      applications: [:plug_logger_json, :httpoison, :edeliver],
      extra_applications: [:logger, :plug_cowboy],
      mod: {My.Application, []}
    ]
  end

Your help will be highly appreciated.