Django Why I can't change from sqlite to postgresql?

I already change the settings.py:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': 'examdatabase',
        'USER': 'examdatabaseuser',
        'PASSWORD': '11111',
        'HOST': '127.0.0.1',
        'PORT': '5432',
    }
}

and I also install postgresql and database:

sudo apt-get install python-psycopg2
sudo apt-get install postgresql

sudo su - postgres 
createuser --superuser examdatabaseuser
psql
\password examdatabaseuser
createdb examdatabase

And I delete the sqlite database file in my project, but when I python manage.py runserver it still use sqlite. Why?

Related info: I use virtualenv (/home/py3env). Ubuntu Server 14.04 LTS.