Upgrade gunicorn version and change worker type

Upgrade gunicorn to 19.9.0 and switch to an asynchronous
worker class

config/gunicorn_config.py

@@ -17,8 +17,8 @@ bind = "127.0.0.1:{port}".format(port=GUNICORN_PORT)
 # Number of workers = 2*CPU + 1 (recommendation from Gunicorn documentation)
 workers = multiprocessing.cpu_count()*2 + 1
 
-# Worker type
-worker_type = 'sync'
+# Worker class
+worker_class = 'gevent'
 
 # Max requests settings - a worker restarts after handling this many
 # requests. May be useful if we have memory leak problems.

requirements.txt

@@ -10,7 +10,7 @@ django-user-sessions==1.6.0
 djangorestframework==3.9.0
 dnspython==1.15.0
 flake8==3.5.0
-gunicorn==19.7.1
+gunicorn[gevent]==19.9.0
 html5lib==1.0.1
 ipdb==0.10.2
 ipython==5.5.0