Add Xcode 8 support: rename "OS X" to "macOS" (#5385)
Add Xcode 8 support: rename "OS X" to "macOS" (#5385)

* OS X -> macOS

Most (but not all) of these changes are cosmetic. Xcode 8 changes the platform string to "macOS" (when used in a destination specification) and also uses macOS as PLATFORM_DISPLAY_NAME.

* Use "OS X" for Xcode 7 and earlier, "macOS" for Xcode 8 and later

* Use Helper to determine Xcode version

fastlane/watchbuild