Add Travis CI shield

Author: Antonis

README.md

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 RestComm Mobile - Android SDK
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+[![CI Status](http://img.shields.io/travis/RestComm/restcomm-android-sdk.svg?style=flat)](https://travis-ci.org/RestComm/restcomm-android-sdk)
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 Android Mobile SDK to easily integrate communication features (messaging, presence, voice, video, screensharing) based on [RestComm](http://restcomm.com/) into native Mobile Applications.
 
 Quick Start guide