RN: Use `OS=latest` for Xcode in CI

Summary:
Changes React Native CI (both Sandcastle and Circle CI) to use `OS=latest` instead of a hardcoded version number.

This reduces the number of hardcoded configurations needed to be updated for new Xcode versions by one.

Changelog:
[Internal]

Reviewed By: fkgozali

Differential Revision: D29013193

fbshipit-source-id: 3c081de1b74a130867b2a599d3e7333846c96cf7