Admit application inference in README

[EpicOrange]

The README installation defines the :applications key which disables application inference from Elixir 1.4. But defining the :extra_applications key adds to the inferred applications without disabling inference, which would be nice! We probably want something like

def application do
  [ extra_applications: extra_applications(Mix.env) ]
end

defp extra_applications(:dev), do: [:remix]
defp extra_applications(_all), do: []

Maybe even just

def application do
  [ extra_applications: case Mix.env do :dev -> [:remix]; _ -> [] end ]
end