switch QUIT and TERM signal

With this change, when gunicorn receives a QUIT all the workers are
killed immediately and exit and TERM is used for the graceful shutdown.

Note: the old behaviour was based on the NGINX but the new one is more
correct according the following doc:

https://www.gnu.org/software/libc/manual/html_node/Termination-Signals.html

also it is complying with the way the signals are sent by heroku:

https://devcenter.heroku.com/articles/python-faq#what-constraints-exist-when-developing-applications-on-heroku