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/last_fm/artist_test.exs

@@ -8,7 +8,7 @@ defmodule LastFm.ArtistTest do
       api_response =
         @api_response_path
         |> File.read!()
-        |> Jason.decode!()
+        |> JSON.decode!()
         |> Map.get("artist")
 
       assert %LastFm.Artist{

test/last_fm/track_test.exs

@@ -8,7 +8,7 @@ defmodule LastFm.TrackTest do
       api_response =
         @api_response_path
         |> File.read!()
-        |> Jason.decode!()
+        |> JSON.decode!()
         |> get_in(["recenttracks", "track"])
 
       assert [