Calabash-ios XCode 5.1 support

- Added the support for selecting the correct simulators when running calabash-ios on iOS 7. (i.e using `DEVICE_TARGET` for selecting a simulator)
- Also kept the support for running on iOS 6

Notes:
- I have gone with the notion of running 4-inch simulators for iOS 7 but maybe we need to have more options on what simulator we can pick.