Update gunicorn to 19.10.0, fix failing build

[lbeaufort]

Summary (required)

• Resolves [Snyk: Medium Severity] HTTP Request Smuggling (Due 3/15/2020) #466
• Resolves Fix failing build #468

Update gunicorn to 19.10.0. See https://snyk.io/vuln/SNYK-PYTHON-GUNICORN-541164
Update GitPython to 2.1.15 due to breaking change in gitdb2 (see gitpython-developers/GitPython#983)

How to test the changes locally

• Gunicorn is only used in production, so you need to deploy to dev to test

Impacted areas of the application

List general components of the application that this PR will affect:

• More info about gunicorn: https://gunicorn.org/
• See https://github.com/fecgov/fec-eregs/blob/develop/manifest_prod.yml

[jason-upchurch]

@pkfec just copying you as a potential reviewer--probably just fine for one reviewer but a chance for more eyes. You and I can coordinate if needed.

[jason-upchurch]

Thanks for doing this work @lbeaufort, looks good. Up to you if you want to squash to a single commit. Test steps included snyk test to verify vulnerability is resolved and manual deploy using tasks.py then clicking around. If you want to merge as is, let me know. Thanks!

[lbeaufort]

@jason-upchurch the commits aren't related, so I'd prefer to keep them separate. Thanks for reviewing!