Update README.md

Author: NiteshKant

README.md

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 RxNetty
 =======
-![Build Status](https://api.travis-ci.org/ReactiveX/RxNetty.svg?branch=0.4.x)
+<a href='https://travis-ci.org/ReactiveX/RxNetty/builds?branch=0.4.x'><img src='https://travis-ci.org/ReactiveX/RxNetty.svg?branch=0.4.x'></a>
 
 Reactive Extension (Rx) Adaptor for Netty