Where possible, use built-in JSON instead of Jason

Note that it's not possible to replace any call where we encode and
pretty print, as JSON doesn't have such functionality.

test/support/fixtures/last_fm_fixtures.ex

@@ -4,6 +4,6 @@ defmodule LastFm.Fixtures do
   def artist_get_info do
     Path.join([@fixtures_folder, "artist.getinfo.json"])
     |> File.read!()
-    |> Jason.decode!()
+    |> JSON.decode!()
   end
 end