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".

lib/puma/configuration.rb

@@ -258,8 +258,10 @@ module Puma
       # requests and +max+ the maximum.
       #
       def threads(min, max)
+        min = Integer(min)
+        max = Integer(max)
         if min > max
-          raise "The minimum number of threads must be less than the max"
+          raise "The minimum (#{min}) number of threads must be less than the max (#{max})"
         end
 
         @options[:min_threads] = min