Use integers when comparing thread counts

When setting min & max via ENV var (PUMA_THREAD_COUNT="6:12") one will
get a RuntimeError, because "6" > "12".

```ruby
min, max = ENV.fetch("PUMA_THREAD_COUNT", "4:8").split(":")
# or
min, max = ENV.fetch("PUMA_THREAD_COUNT", "4:8").split(":").map(&:to_i)
# but the latter feels funny to me

threads min, max
```