Add Travis badge

README.md

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 [![Join the chat at https://gitter.im/kr15h/ofxPiMapper](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/kr15h/ofxPiMapper?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
+[![Build Status](https://travis-ci.org/kr15h/ofxPiMapper.svg?branch=master)](https://travis-ci.org/kr15h/ofxPiMapper)
+
 Projection mapping addon for openFrameworks that runs on the Raspberry Pi.
 
 The project started as master's thesis project by [Krisjanis Rijnieks](https://rijnieks.com) at the [Helsinki Media Lab](https://medialab.aalto.fi/). Currently undergoing changes. Expect the API to be incompatible with your projects from the past or present-day.