Modulus operator does not create uniformly distributed random numbers

[p-salido]

https://github.com/fetlife/rollout/blob/master/lib/rollout.rb#L91

https://stackoverflow.com/questions/10984974/why-do-people-say-there-is-modulo-bias-when-using-a-random-number-generator#10984975

[reneklacan]

There's already similar issue #58

We know it's not perfect, do you have suggestion how to address this? :)