Revert "`guncorn -k tornado` doesn't work."

This reverts commit 5e15b67743d99b14ef7508466285b5e37a5c6b99.
Revert gunicorn to 18.0

gunicorn 19.0 has regression about tornado worker.
gunicorn+tornado is easiest HTTP server supporting keep-alive for PyPy.
So back to 18.0 until 19.1 is released.