Use :host strategy for iOS 8 GM devices

`UIAApplication.preferencesValueForKey()` is throwing errors on iOS 8 GM. [1]

Launcher checks the the device target and if it is an iOS 8 device, it chooses `:host` as the default uia strategy.

Users can still override the default by passing `{:uia_strategy => ...}`.

- [1] http://openradar.appspot.com/radar?id=5891145586442240