Always start iOS-simulator with 7.0 SDK

Added `--retina` to force switch to 7.0 when iOS-simulator is already running in non-retina with an older SDK. When it already runs retina but with an older SDK this won't help and I see no other way to then force 7.0 to be used.
Replace '7.0' with version provided by buildlib.
Merge pull request #121 from FokkeZB/patch-2

Always start iOS-simulator with latest SDK