speed up Integer#multiple_of?

ruby 2.0.0p247
Rehearsal ---------------------------------------
old   1.300000   0.010000   1.310000 (  1.304970)
new   1.110000   0.000000   1.110000 (  1.110088)
------------------------------ total: 2.420000sec

          user     system      total        real
old   1.300000   0.000000   1.300000 (  1.297435)
new   1.110000   0.000000   1.110000 (  1.110639)