Downgrade gunicorn for tornado worker

Gunicorn 19.0 has regression. Tornado worker doesn't works with it.
Tornado is best worker for PyPy (speed and keep-alive).
Until Gunicorn 19.1 is released, downgrade to 18.0.
Gunicorn+Tornado is for PyPy. Py2/Py3 can use Gunicron 19.0.