Bump gunicorn from 18.0 to 19.5.0

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 18.0 to 19.5.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@18.0...19.5.0)

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

requirements.txt

@@ -3,7 +3,7 @@ asgiref==3.1.2
 channels==1.1.8
 daphne==1.4.2
 Django==2.1.4
-gunicorn==18.0
+gunicorn==19.5.0
 whitenoise
 dj-database-url
 psycopg2