update README.md to link to travis-ci's build page

Author: jimjh

README.md

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 # Kafka Python client
 
-![travis](https://travis-ci.org/mumrah/kafka-python.png)
+[![Build Status](https://travis-ci.org/mumrah/kafka-python.png)](https://travis-ci.org/mumrah/kafka-python)
 
 This module provides low-level protocol support for Apache Kafka as well as
 high-level consumer and producer classes. Request batching is supported by the