Gunicorn 'ImportError: No module named app.wsgiapp' on heroku

I cannot run gunicorn on heroku with simple flask app.
The application is really simple. This is app.py:

app = Flask(__name__)

@app.route("/")
def say_hello(url):
    return "Hello"


if __name__ == "__main__":
    port = int(os.environ.get('PORT', 8888))
    app.run(host='0.0.0.0',port=port)

The app works fine through flask test server on heroku, but when I switch to use gunicorn, it crashes with:

ImportError: No module named app.wsgiapp

My requirements.txt:

Flask==0.8
gevent==0.13.7
gunicorn==0.13.2

I've tried different versions of gunicorn from 0.13.7 to 0.14.6 with no success.

Procfile:

web: gunicorn app:app -w 4 -b 0.0.0.0:$PORT

Running this command:

heroku logs

gives this:

←[33m2012-08-09T21:08:02+00:00 app[web.1]:←[0m ImportError: No module named app.
wsgiapp ←[33m2012-08-09T21:08:02+00:00 app[web.1]:←[0m     entry = __import__(self.modul
e_name, globals(),globals(), ['__name__'])

Any help please?