Bump gunicorn from 19.9.0 to 22.0.0

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.9.0 to 22.0.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.9.0...22.0.0)

---
updated-dependencies:
- dependency-name: gunicorn
  dependency-version: 22.0.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements.server.txt

@@ -5,7 +5,7 @@ click==6.7
 Flask-RESTful==0.3.4
 Flask-WTF==0.12
 Flask==0.12.2
-gunicorn==19.9.0
+gunicorn==22.0.0
 itsdangerous==0.24
 Jinja2==2.11.3
 MarkupSafe==1.1.1