Change baseDir in config.combine to match docs

I've been trying out elixir and I think it's great! However I got stuck when trying to use `.script(['js/app.js'])`. The documentation seems to imply the default base directory for `.script()` is `resources/assets` but the `config.combine` function sets the default to `'public'`. So I've changed it to use `config.assetsDir` instead which is defined as `'resources/assets'`.