Updated README.md, now Swift 3 only
Updated README.md, now Swift 3 only

The swift-2.2 branch points to the pre-swift3 commit, new features
from master will be backported on request.

stephanmantler/SwiftyGPIO,colemancda/SwiftyGPIO,uraimo/SwiftyGPIO,uraimo/SwiftyGPIO,stephanmantler/SwiftyGPIO