Add support for redis-rb >= 4.3

[marcelolx]

Follow-up of #346

When that PR gets merged we need to add support for redis-rb >= 4.3 which will return an integer value for exists while prior versions return a boolean value.

Fixing the warning described in original PR #335 will be made in a future PR and will be included only in sidekiq-scheduler v4 as described here #345 (comment)

[juan-apa]

1 and true evaluate to a truthy value, so doing r.exists(key) ? true : false would do the trick here

[marcelolx]

@juan-apa Yeah, but 0 also will evaluate to a truthy value

irb(main):001:0> true ? 'A' : 'B'
=> "A"
irb(main):002:0> 1 ? 'A' : 'B'
=> "A"
irb(main):003:0> 0 ? 'A' : 'B'
=> "A"
irb(main):004:0> false ? 'A' : 'B'
=> "B"

[juan-apa]

oh wow. I mean, makes sense, but at the same time it doesn't. It's fine by me if you keep it with the case then.