Update Swift version badge.

Author: Martin Ø. Christensen

README.md

@@ -23,7 +23,7 @@
 
 <p align="center">
     <a href="https://developer.apple.com/swift/" target="_blank">
-        <img src="https://img.shields.io/badge/Swift-3.0-orange.svg?style=flat" alt="Swift 3.0">
+        <img src="https://img.shields.io/badge/Swift-4.1-orange.svg?style=flat" alt="Swift 4.1">
     </a>
     <a href="https://developer.apple.com/swift/" target="_blank">
         <img src="https://img.shields.io/badge/Platforms-OS%20X%20%7C%20Linux%20-lightgray.svg?style=flat" alt="Platforms OS X | Linux">