Add an upper limit to the version of python-redis we can use
    
    Otherwise a large number of our tests is failing on Jenkins with the
    error:
    ````
    Traceback (most recent call last):
      File "/root/pagure/.tox/py27-flask011-ci/lib/python2.7/site-packages/mock/mock.py", line 1305, in patched
        return func(*args, **keywargs)
      File "/root/pagure/tests/test_pagure_flask_dump_load_ticket.py", line 61, in test_dumping_reloading_ticket
        user='pingou',
      File "/root/pagure/tests/../pagure/lib/query.py", line 1809, in new_issue
        pagure.lib.git.update_git(issue, repo=repo)
      File "/root/pagure/tests/../pagure/lib/git.py", line 188, in update_git
        _maybe_wait(queued)
      File "/root/pagure/.tox/py27-flask011-ci/lib/python2.7/site-packages/mock/mock.py", line 1062, in __call__
        return _mock_self._mock_call(*args, **kwargs)
      File "/root/pagure/.tox/py27-flask011-ci/lib/python2.7/site-packages/mock/mock.py", line 1128, in _mock_call
        ret_val = effect(*args, **kwargs)
      File "/root/pagure/tests/test_pagure_flask_dump_load_ticket.py", line 43, in <lambda>
        mw.side_effect = lambda result: result.get()
      File "/root/pagure/.tox/py27-flask011-ci/lib/python2.7/site-packages/celery/result.py", line 995, in get
        raise self.result
    AttributeError: 'UnixDomainSocketConnection' object has no attribute '_buffer_cutoff'
    ````
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>