Add Travis CI Icon

Author: zhigang1992

README.md

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 #SwiftyJSON [中文介绍](http://tangplin.github.io/swiftyjson/)
 
+[![Travis CI](https://travis-ci.org/SwiftyJSON/SwiftyJSON.svg?branch=master)](https://travis-ci.org/SwiftyJSON/SwiftyJSON)
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 SwiftyJSON makes it easy to deal with JSON data in Swift.
 
 1. [Why is the typical JSON handling in Swift NOT good](#why-is-the-typical-json-handling-in-swift-not-good)