Added :building_number method to Faker::Address module

This mimics the behavior of the original [faker](https://github.com/stympy/faker) gem.

I noticed that `ffaker` didn't have this method after swapping in `ffaker` for `faker` on a project, which was invoking `Faker::Address.building_number` which subsequently started throwing `NoMethodError`'s.  Here is the [source location](https://github.com/stympy/faker/blob/v1.1.2/lib/faker/address.rb#L22).

There may be more logical applications for the `building_number` method than just using it in `Faker::Address.street_address`, but I wasn't certain.  That seemed to be the most obvious location for usage.