minor improvements to django example

examples/wsgi/django_example/requirements.txt

@@ -0,0 +1,8 @@
+Django==1.11.1
+enum-compat==0.0.2
+eventlet==0.21.0
+greenlet==0.4.12
+python-engineio
+python-socketio
+pytz==2017.2
+six==1.10.0

examples/wsgi/django_example/socketio_app/views.py

@@ -1,7 +1,7 @@
 # set async_mode to 'threading', 'eventlet', 'gevent' or 'gevent_uwsgi' to
 # force a mode else, the best mode is selected automatically from what's
 # installed
-async_mode = 'threading'
+async_mode = None
 
 import os