Add json_polyfill when required

Author: kipcole9

mix.exs

@@ -40,7 +40,15 @@ defmodule Image.OCR.MixProject do
       {:elixir_make, "~> 0.8", runtime: false},
       {:ex_doc, "~> 0.34", only: :dev, runtime: false},
       {:dialyxir, "~> 1.4", only: [:dev, :test], runtime: false}
-    ]
+    ] ++ maybe_json_polyfill()
+  end
+
+  defp maybe_json_polyfill do
+    if Code.ensure_loaded?(:json) do
+      []
+    else
+      [{:json_polyfill, "~> 0.2 or ~> 1.0"}]
+    end
   end
 
   defp description do