4.0.0: [README] Update version numbers in README to 4.0

Breaking changes

• Swift 4 support

3.0.0

Breaking changes

• Support for Swift 3

2.0.2

Explicitly specify Swift 2.3 so that Xcode 8 can compile the module (#6)

2.0.1

This tag shares Synchronized.xcscheme for compatibility with Carthage.