Update to RxSwift RC

- [x] `just` -> `Observable.just`
- [x] `Variable` is no longer a direct `ObservableType`
- [x] `empty()` -> `Observable.empty`
- [x] `MainScheduler.sharedInstance` -> `MainScheduler.instance`
- [x] `create` -> `Observable.create`
- [x] `combineLatest` -> `Observable.combineLatest`
