roll back to a version tagged as "previous"
git push -f heroku previous:master
roll back three commits:
git push -f heroku HEAD~3:master
roll back to a specific SHA. Does NOT affect your current working branch!
git push -f heroku a3fe67b:master
NOTE: Does not rollback database. Must do
heroku run rake db:migrate VERSION=20090901123000
git push -f heroku old-version:master