I'm guessing (didn't double check) that the is.gd URL's are not expanded in the Twitter archive under expanded_url. That means the script would have to fetch the minified URL and see where the redirect points.

In that case I suggest creating a separate JSON / CSV file to track the mapping between short and long URL's. That way if you ever have to rerun the script, it won't hammer the URL shortener again needlessly, and if the shortener ever disappears, you still have the mapping.