Update README for Swift 4

Author: zachwaugh

README.md

@@ -11,7 +11,7 @@
 
 ## Requirements
 
-Turbolinks for iOS is compatible with Xcode 8/Swift 3.0. It requires iOS 8 or higher. The Swift language requires dependencies to be compiled with the same version as the target that includes them. If you need Swift 2.3 compatibility, use the `v1.0.3` release, which is the last release to support Swift 2.3.
+Turbolinks for iOS is written in Swift 4.0 and requires Xcode 9. It should also work with Swift 3.2 as well. It currently supports iOS 8 or higher, but we'll most likely drop iOS 8 support soon.
 
 Web views are backed by [WKWebView](https://developer.apple.com/library/ios/documentation/WebKit/Reference/WKWebView_Ref/) for full-speed JavaScript performance.
 
@@ -305,4 +305,4 @@ Please note that this project is released with a [Contributor Code of Conduct](C
 
 ---
 
-© 2016 Basecamp, LLC
+© 2017 Basecamp, LLC