[README] Update version numbers in README to 4.0

[ci skip]

[3.0.0] Bump the podspec version to 3.0.0 for Swift 3 support

[2.0.2] Explicitly ask for Swift 2.3 for Xcode 8 compatibility

[2.0.1] Add Carthage support

This commit will be tagged as 2.0.1.

[Pod] Bump version to 2.0.0 since only Xcode 6.3+ is supported

Older versions of Xcode don't support the `@noescape` attribute, so consider this a breaking change. If you are using Synchronized and have upgraded to Xcode 6.3 / Swift 1.2, you can upgrade to Synchronized 2.0.0 without any worries.

[CocoaPod] Add a podspec for 1.0.0

Validated with `pod lib lint`