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.

config/requirements-py3.txt

@@ -8,7 +8,7 @@ ujson==1.33
 # uwsgi is released too often to stick on single version.
 uwsgi
 
-gunicorn==19.0
+gunicorn==18.0
 meinheld==0.5.6
 
 # Tornado

config/requirements-pypy.txt

@@ -9,7 +9,7 @@ PyMySQL==0.6.2
 # uwsgi is released too often to stick on single version.
 uwsgi
 
-gunicorn==19.0
+gunicorn==18.0
 meinheld==0.5.6
 
 # Tornado

config/requirements.txt

@@ -9,7 +9,7 @@ gevent==1.0.1
 # uwsgi is released too often to stick on single version.
 uwsgi
 
-gunicorn==19.0
+gunicorn==18.0
 meinheld==0.5.6
 
 # Tornado

flask/setup_pypy.py

@@ -15,6 +15,7 @@ def start(args, logfile, errfile):
     proc = subprocess.Popen([
         bin_dir + "/gunicorn",
         "app:app",
+        "-k", "tornado",
         "-b", "0.0.0.0:8080",
         '-w', str(NCPU*3),
         "--log-level=critical"],