Fix map_diff version spec in tests

Author: filmor

rustler_tests/mix.exs

@@ -19,7 +19,7 @@ defmodule RustlerTest.Mixfile do
   defp deps do
     [
       {:rustler, path: "../rustler_mix", runtime: false},
-      {:map_diff, version: "~> 1.3"}
+      {:map_diff, "~> 1.3"}
     ]
   end
 end