Update EC2 price scraping script

I tried to run EC2 price scraping script before the release (``contrib/scrape-ec2-sizes.py``), but it seems the format in the JSON and JavaScript files has changed so the script doesn't give expected results anymore.

We need to update the script to correctly handle the new format.

Start with contrib/scrape-ec2-sizes.py and run it before examining how the current JSON and JavaScript formats differ from what the script expects. Done means the script handles the new format and produces the expected EC2 price results; no test file is mentioned in the issue.

索引モデルが issue の本文から書いたものです。