Redis backend with race check bug

[radandrei]

This is a corner case for the issue addressed here: #29.
This occurs when the version fetched by State() is deleted before SetState() is executed. That will cause an error: got [1 1 OK OK] from redis, expected [<version_before_remove>+1 1 OK OK] raised by checkResponseFromRedis.

Potential solution is to always expect version to be in initial state: #42

[mennanov]

Fixed in #42