tox: run tests with redis<3

Version 3 isn't compatible with our old celery 3.1.25 and break our tests, so use version 2 for now

tox.ini

@@ -7,6 +7,7 @@ deps =
   pbr>=1.4
   pifpaf
   mock
+  redis<3
 commands =
   pifpaf run redis --port 6380 -- {envpython} -m pytest {posargs:test}