Allow easy switching of RDF.ex packages during development

mix.exs

@@ -63,7 +63,7 @@ defmodule JSON.LD.Mixfile do
 
   defp deps do
     [
-      {:rdf, "~> 0.9"},
+      rdf_ex_dep(:rdf, "~> 0.9"),
       {:jason, "~> 1.2"},
       {:httpoison, "~> 1.6"},
       {:dialyxir, "~> 1.1", only: :dev, runtime: false},
@@ -73,6 +73,13 @@ defmodule JSON.LD.Mixfile do
     ]
   end
 
+  defp rdf_ex_dep(dep, version) do
+    case System.get_env("RDF_EX_PACKAGES_SRC") do
+      "LOCAL" -> {dep, path: "../#{dep}"}
+      _ -> {dep, version}
+    end
+  end
+
   defp dialyzer do
     [
       plt_file: {:no_warn, "priv/plts/dialyzer.plt"}