add travis build status badge

Author: freyfogle

README.md

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+[![Build Status](https://travis-ci.org/OpenCageData/perl-Geo-Coder-OpenCage.svg?branch=master)](https://travis-ci.org/OpenCageData/perl-Geo-Code-OpenCage)
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 # Geo::Coder::OpenCage
 
 This module provides an interface to the OpenCage geocoding service. For more details see http://geocoder.opencagedata.com/api.html