class MockRedis

TODO: Implement the following commands

* xgroup
* xreadgroup
* xack
* xpending
* xclaim
* xinfo
* xtrim
* xdel

TODO: Complete support for

* xtrim
    - `approximate: true` argument is currently ignored
* xadd
    - `approximate: true` argument (for capped streams) is currently ignored

For details of these commands see

* https://redis.io/topics/streams-intro
* https://redis.io/commands#stream

Defines the gem version.