Bump gunicorn from 19.10.0 to

                      20.0.4

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.10.0 to 
                      20.0.4.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.10.0...20.0.4)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -1,7 +1,8 @@
 beautifulsoup4==4.9.2
 elasticsearch==5.5.3
 flask-cors==3.0.9
-gunicorn==19.10.0
+gunicorn==
+                      20.0.4
 nested-lookup==0.2.21
 pylint==2.3.1
 python-memcached==1.59