Get the Heroku db as detailed here: http://devcenter.heroku.com/articles/pgbackups#exporting_via_a_backup
heroku pg:backups capture
heroku pg:backups --app <you-app-name> #=> <backup_num>
heroku pg:backups public-url <backup_num> #=> <backup_url>
curl -o db/latest.dump <backup_url>
echo 'db/latest.dump' >> .gitignore # if not already done