fix redis exists deprecation warning

[Wolfer]

With redis 4.2.5 I get deprecated text at startup

Redis#exists(key)` will return an Integer in redis-rb 4.3. `exists?` returns a boolean, you should use it instead. To opt-in to the new behavior now you can set Redis.exists_returns_integer =  true. To disable this message and keep the current (boolean) behaviour of 'exists' you can set `Redis.exists_returns_integer = false`, but this option will be removed in 5.0. (/usr/local/rvm/gems/ruby-2.6.3/gems/sidekiq-scheduler-3.0.1/lib/sidekiq-scheduler/redis_manager.rb:99:in `block in schedule_exist?')

Method exists? was added recently and most likely will not be available to everyone(https://github.com/redis/redis-rb/blame/master/lib/redis.rb#L601)

[coveralls]

Coverage decreased (-0.1%) to 97.86% when pulling 8421e5c on Wolfer:patch-1 into 41990aa on moove-it:master.

[weilandia]

We may want to also add the change here: https://github.com/moove-it/sidekiq-scheduler/blob/41990aafa80a0f458e988733189fbf4b07e0894a/spec/support/store.rb#L61

[Wolfer]

We may want to also add the change here:

https://github.com/moove-it/sidekiq-scheduler/blob/41990aafa80a0f458e988733189fbf4b07e0894a/spec/support/store.rb#L61

done

[marcelolx]

@Wolfer Soon as possible I'll take a look on this, the tests are broken but this is already fixed here #339

[elpic]

LGTM

[marcelolx]

Thanks @Wolfer