speed up Integer#multiple_of?
ruby 2.0.0p247
Rehearsal ---------------------------------------
old   0.650000   0.000000   0.650000 (  0.654919)
new   0.560000   0.000000   0.560000 (  0.555761)
------------------------------ total: 1.210000sec

          user     system      total        real
old   0.650000   0.000000   0.650000 (  0.646276)
new   0.560000   0.000000   0.560000 (  0.560981)