TT#96400 requirements: set supported version for celery

* refresh Docker file
* https://docs.celeryproject.org/en/stable/whatsnew-5.0.html
> Celery 4.x will be supported until the 1st of August, 2021.`

Change-Id: I4eb6565a620a1cae9aa47d4e88e47a9ba123859f

requirements/common.txt

@@ -1,4 +1,4 @@
-celery
+celery>4.4,<5.0
 Django==1.11.29
 django-appconf
 django-assets

t/Dockerfile

@@ -5,7 +5,7 @@ FROM docker.mgm.sipwise.com/sipwise-buster:latest
 # is updated with the current date. It will force refresh of all
 # of the base images and things like `apt-get update` won't be using
 # old cached versions when the Dockerfile is built.
-ENV REFRESHED_AT 2020-04-02
+ENV REFRESHED_AT 2020-10-12
 
 # test execution; we need the backport of python3-junitxml from our own
 # repository since it's not part of Debian/buster