Avoid 2 Xcode errors and "view is not in the window hierarchy error on iOS8"

Hi,
I'm not sure about this fix (and I'm new with Swift), it could be better to add a public property "receiverController" to RateMyApp class. This controller could be set before call the "trackAppUsage".
In all case, I think this fallback is required.

Let me know.

- [ ] 2 Xcode errors
- [ ] view is not in the window hierarchy error on iOS8
- [ ] Usage of NSLocalizedString

++