Add Time#in_milliseconds

This method is implemented on the [Numeric's time extension](https://github.com/rails/rails/blob/f1eddea1e3f6faf93581c43651348f48b2b7d8bb/activesupport/lib/active_support/core_ext/numeric/time.rb), I believe that it should be available on `Time` as well.

Thanks @bcjordan for pointing this out.