Completely remove devise secret key from git repository

Took me long enough to figure this out, so I might as well write it down. I'm replacing the secret key with an environment variable:

git filter-branch --tree-filter "sed -i -e 's/'\''123123123123123123123123123123123123123'\''/ENV[\"DEVISE_SECRET_KEY\"]/g' config/initializers/devise.rb || true"