[Security] Bump gunicorn from 0.17.4 to 19.5.0

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 0.17.4 to 19.5.0. **This update includes a security fix.**
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@0.17.4...19.5.0)

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

requirements.txt

@@ -9,7 +9,7 @@ letter==0.5
 cffi==1.7.0
 coverage==3.6
 dj-database-url==0.2.1
-gunicorn==0.17.4
+gunicorn==19.5.0
 psycopg2==2.8.3
 dj-static==0.0.6
 django-axes==1.7.0