Allow Safari to ungzip assets served by WhiteNoise

Whitenoise 4.x has a bug when used with Django 3+ where the former
serves a `Content-Disposition` header that makes Safari think that it
doesn't need to (or can't) ungzip the response, causing the assets to
fail to parse. This is described in (slightly) more detail here:
evansd/whitenoise#241, and was resolved in
WhiteNoise 5.

requirements.txt

@@ -4,6 +4,6 @@ boto==2.49.0
 gunicorn~=21.2
 pyScss~=1.4
 eventlet~=0.36.1
-whitenoise~=4.1.4
+whitenoise~=6.0
 greenlet~=3.0
 tzdata