To run, download the raw file. Then run:
ruby convert_ignore.rb your_new_report.json old_brakeman.ignore > new_brakeman.ignore
Then test by pointing Brakeman at the new file and checking the number of ignored warnings matches expected:
brakeman -i new_brakeman.ignore
Assuming all is well, copy the new ignore file to the old location.
Please comment here with questions/problems or open an issue.