Jeffrey Way from the Laracasts forum:
So, now, the default locations will be resources/assets/js and resources/assets/css. If/When you update Elixir, be sure to check your Gulpfile to see if you need to update any paths.
He also points out that you can keep the old style by passing the path as the third parameter:
mix.scripts(['one.js', 'two.js'], null, './resources/js');
Read the full forum thread for more information.