Do not raise NotImplementedError if changed file is not in local_assets_...

If there are changed files excluded via the ignore_files setting the watch method is raising a NotImplementedError. In my opinion this is a bug so i changed the if clause to better behave in such cases. If you confirm this as a bug and not a feature i would be happy if you will merge this.
what if we were to simply `next unless local_assets_list.include?(filename)`

That way we can reduce the amount of indentation that's kinda noisy. I *think* that should keep the functionality we want, right?