Pin redis<3

The latest version of redis-py breaks the unit tests:

=================================== FAILURES ===================================
_________________________________ test_nagios __________________________________

capsys = <_pytest.capture.CaptureFixture object at 0x119c9e390>
monkeypatch = <_pytest.monkeypatch.MonkeyPatch object at 0x119d67048>
socket_enabled = None
starter = <class 'gwcelery.tests.process.starter.<locals>.Starter'>
tmpdir = local('/private/var/folders/cs/pwq97bws17d2p8qcp410gs7c84mbdz/T/pytest-of-lpsinger/pytest-7/test_nagios0')

    def test_nagios(capsys, monkeypatch, socket_enabled, starter, tmpdir):
        # no broker

        unix_socket = str(tmpdir / 'sock')
        monkeypatch.setenv('CELERY_BROKER_URL', 'redis+socket://' + unix_socket)

        with pytest.raises(SystemExit) as excinfo:
            app.start(['gwcelery', 'nagios'])
        assert excinfo.value.code == nagios.NagiosPluginStatus.CRITICAL
        out, err = capsys.readouterr()
        assert 'CRITICAL: No connection to broker' in out

        # broker, no worker

        redis_server = find_executable('redis-server')
        if redis_server is None:
            pytest.skip('redis-server is not installed')
        starter.exec_process(
            [redis_server, '--port', '0', '--unixsocket', unix_socket,
             '--unixsocketperm', '700'], timeout=10,
            magic_words=b'The server is now ready to accept connections')

        with pytest.raises(SystemExit) as excinfo:
            app.start(['gwcelery', 'nagios'])
        assert excinfo.value.code == nagios.NagiosPluginStatus.CRITICAL
        out, err = capsys.readouterr()
        assert 'CRITICAL: Not all expected queues are active' in out

        # worker, no tasks

        starter.python_process(
            args=(['gwcelery', 'worker', '-l', 'info',
                   '-Q', 'celery,exttrig,openmp,superevent'],),
            target=app.start, timeout=10, magic_words=b'ready.')

        with pytest.raises(SystemExit) as excinfo:
            app.start(['gwcelery', 'nagios'])
        assert excinfo.value.code == nagios.NagiosPluginStatus.CRITICAL
        out, err = capsys.readouterr()
>       assert 'CRITICAL: Not all expected tasks are active' in out
E       AssertionError: assert 'CRITICAL: Not all expected tasks are active' in ' \n -------------- celery@gs66-wintermute v4.2.1 (windowlicker)\n---- **** ----- \n--- * ***  * -- Darwin-18.2.0-x86_...nCRITICAL: Not all expected queues are active\nAssertionError: Missing queues: openmp, exttrig, celery, superevent\n\n'

gwcelery/tests/test_nagios.py:60: AssertionError