Bump gunicorn from 19.1.1 to 19.5.0

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.1.1 to 19.5.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.1.1...19.5.0)

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

requirements.txt

@@ -5,7 +5,7 @@ django-appconf==0.6
 django-compressor==1.4
 djangorestframework==3.11.2
 drf-nested-routers==0.9.0
-gunicorn==19.1.1
+gunicorn==19.5.0
 six==1.8.0
 static3==0.5.1
 wsgiref==0.1.2