fix bm_nsieve.rb on 1.9.2
fix bm_nsieve.rb on 1.9.2

acangiano/ruby-benchmark-suite,acangiano/ruby-benchmark-suite,acangiano/ruby-benchmark-suite,acangiano/ruby-benchmark-suite,acangiano/ruby-benchmark-suite,acangiano/ruby-benchmark-suite,acangiano/ruby-benchmark-suite,acangiano/ruby-benchmark-suite