Bump gunicorn from 19.7.1 to 19.8.1 in /services/users (#92)

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.7.1 to 19.8.1.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.7.1...19.8.1)

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

services/users/requirements.txt

@@ -3,7 +3,7 @@ Flask-Script==2.0.6
 Flask-SQLAlchemy==2.3.2
 psycopg2==2.7.4
 Flask-Testing==0.7.1
-gunicorn==19.7.1
+gunicorn==19.8.1
 coverage==4.5.1
 flake8==3.5.0
 flask-cors==3.0.4