Update Carthage badge

Carthage/Carthage#179
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README.md

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 # Base32 for Swift on OS X and iOS
 [![MIT Lincese](http://img.shields.io/badge/license-MIT-blue.svg?style=flat)](LICENSE)
-[![Carthage Compatibility](https://img.shields.io/badge/carthage-✓-f2a77e.svg?style=flat)](https://github.com/Carthage/Carthage/)
+[![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)
 [![CI Status](http://img.shields.io/travis/norio-nomura/Base32.svg?style=flat)](https://travis-ci.org/norio-nomura/Base32)
 
 Base32 is a [Base32](https://tools.ietf.org/html/rfc4648) implementation for Swift.