Use RLMArray directly instead of copying into Swift array

As discussed on [Twitter](https://twitter.com/simjp/status/501423390897364993), it's more performant (and less code) to use Realm's `RLMArray` directly rather than copy those values into a Swift Array.

The one advantage you lose is that RLMArray isn't generic, so you have to cast objects you take out of it to `City`. We're working on improving our Swift interface, which will include generic support for RLMArray.