Mitigate rate limiter slow down due to rounding error

Use `to_f` instead of `to_i` to avoid slowing down the rate limiter. Checkout [this ticket](https://github.com/enova/sidekiq-rate-limiter/issues/10) I opened on the sidekiq-rate-limiter gem. Basically, using `to_i` introduces huge rounding errors (technically, a `floor`-ing error) that can significantly slow things down over time.