Update Swift badge in README to 4.2

Author: tsolomko

README.md

@@ -1,6 +1,6 @@
 # BitByteData
 
-[![Swift 4.1](https://img.shields.io/badge/Swift-4.1-blue.svg)](https://developer.apple.com/swift/)
+[![Swift 4.2](https://img.shields.io/badge/Swift-4.2-blue.svg)](https://developer.apple.com/swift/)
 [![GitHub license](https://img.shields.io/badge/license-MIT-lightgrey.svg)](https://raw.githubusercontent.com/tsolomko/BitByteData/master/LICENSE)
 [![Build Status](https://travis-ci.org/tsolomko/BitByteData.svg?branch=develop)](https://travis-ci.org/tsolomko/BitByteData)