Fix getting sim dirs

Rather than using the SDK version to find the correct directory for the simulator files, use the platform version. Otherwise when using a version of Xcode with multiple simulator versions will fail in certain situations.

E.g., with Xcode `5.1.1`, which has an SDK version of `7.1`, if you install, and request, a `6.1` simulator, Appium would try to use `/Users/isaac/Library/Application Support/iPhone Simulator/7.1` as the root for the simulator, rather than `/Users/isaac/Library/Application Support/iPhone Simulator/6.1`.

Fixes #4137.