[Security] Bump gunicorn from 19.4.1 to 19.9.0

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.4.1 to 19.9.0. **This update includes security fixes.**
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.4.1...19.9.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

requirements.txt

@@ -18,7 +18,7 @@ django-taggit==0.19.0
 django-user-accounts==1.3.1
 docutils==0.12
 pinax-eventlog==1.1.0
-gunicorn==19.4.1
+gunicorn==19.9.0
 jsonfield==1.0.3
 kombu==3.0.30
 metron==1.3.7