Revert "fix redis exists deprecation warning"

[marcelolx]

Reverts #335

Ref: #345

[marcelolx]

@juan-apa Just to give you more context, I think the best option to fix #345 is if we check the redis version and then chose which method call, but mock_redis only implements exists? on v0.24 https://github.com/sds/mock_redis/releases/tag/v0.24.0 so we will need to upgrade since we are using v0.19 and between this bumps support for some ruby versions was dropped which I think we should drop too since ruby 2.4 already reached end of life a year ago.