Fix deprecation warning with Redis 4.8.0

[martingregoire]

Fixes the following deprecation warning when using Redis 4.8.0:

Redis#sadd will always return an Integer in Redis 5.0.0.
Use Redis#sadd? instead.

As the return value of redis.sadd is not used, we can simply replace the .sadd call with .sadd?.

see

• https://github.com/redis/redis-rb/blob/4.x/CHANGELOG.md#480

• 700