Here is one more thing I have to take note when messing around with Heroku:
After installing a fresh copy of Xubuntu 13.10 (Saucy Salamander), I created a new virtualenv with all the requirement. I made some changes in the Django app. And, finally I intended to push the updates to Heroku:
$ git push heroku master
But, it said that I don't have permission.
OK, I don't have the right ssh key on my new OS. So, I fixed this by:
1. Generate new pair keys:
$ ssh-keygen -t rsa
2. Add the new key to Heroku:
$ heroku keys:add
Done. Everything's back to normal. I can push code to Heroku again.