Summary:
When multiple simulator runtimes are available, this change will make it possible to specify which one to run. So assuming you have e.g. iOS 9.0 and 9.3 runtimes, all following calls will now work:
```
react-native run-ios --simulator "iPhone 6s (9.0)"
react-native run-ios --simulator "iPhone 6s (9.3)"
react-native run-ios --simulator "iPhone 6s"
```
Closes https://github.com/facebook/react-native/pull/8559
Differential Revision: D3516811
Pulled By: frantic
fbshipit-source-id: c81658f77e482e712293367b13d27e783e538aad