Fix compile error with redis shard v0.15.1

[akadusei]

In src/defense/redis_store.cr:21:11

 21 | end.first.as(Int64)
          ^----
Error: undefined method 'first' for Nil (compile-time type is (Array(Redis::Value) | Nil))

The redis shard introduced a breaking change in v0.15.1; it changed the return type of #multi to a nilable array.

[rodrigopinto]

@akadusei Thanks! Please update the shard dependency and lock accordingly.

[akadusei]

Done; redis shard upgraded to v0.15.