pin gunicorn to git commit; bump eventlet

gunicorn doesn't pin eventlet, but functionally, gunicorn==20.1.0
depends on eventlet<=0.30.2 due to a change in eventlet. Gunicorn have
fixed this compat issue, however, haven't released it. By pinning to a
git commit, we're able to bump eventlet up to 0.33, thus solving a
security advisory. (Note that the security advisory didn't actually
impact us as it only affects websockets, however, it was noisy and
distracting).

Note - pip may have cached the old version of gunicorn. You may need to
run `pip install -r requirements.txt --no-cache-dir` to get the updated
version of gunicorn locally.

requirements.in

@@ -9,8 +9,8 @@ Flask-Migrate==2.7.0
 git+https://github.com/mitsuhiko/flask-sqlalchemy.git@500e732dd1b975a56ab06a46bd1a20a21e682262#egg=Flask-SQLAlchemy==2.3.2.dev20190108
 Flask==1.1.2
 click-datetime==0.2
-eventlet==0.30.2 # pyup: ignore # 0.31 breaks Gunicorn
-gunicorn==20.1.0
+# Should be pinned until a new gunicorn release greater than 20.1.0 comes out. (Due to eventlet v0.33 compatibility issues)
+git+https://github.com/benoitc/gunicorn.git@1299ea9e967a61ae2edebe191082fd169b864c64#egg=gunicorn[eventlet]==20.1.0
 iso8601==0.1.14
 itsdangerous==1.1.0
 jsonschema==3.2.0