Update Swift version badge

Author: devxoul

README.md

@@ -1,7 +1,7 @@
 SafeIndex
 =========
 
-![Swift](https://img.shields.io/badge/Swift-2.1-orange.svg)
+![Swift](https://img.shields.io/badge/Swift-2.2-orange.svg)
 [![Build Status](https://travis-ci.org/devxoul/SafeIndex.svg)](https://travis-ci.org/devxoul/SafeIndex)
 [![CocoaPods](http://img.shields.io/cocoapods/v/SafeIndex.svg)](https://cocoapods.org/pods/SafeIndex)
 [![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)