Bump max version requirement of redis (close snowplow#223)

Author: akachanov

setup.py

@@ -80,7 +80,7 @@
         "pycontracts>=1.7.6,<2.0",
         "celery>=4.0,<5.0",
         "gevent>=1.0.2,<2.0",
-        "redis>=2.9.1,<3.0",
+        "redis>=2.9.1,<4.0",
         "six>=1.9.0,<2.0"
     ],
 )