README.md: add Travis badge

README.md

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 # minmea, a lightweight GPS NMEA 0183 parser library
 
+[![Build Status](https://travis-ci.org/cloudyourcar/minmea.svg?branch=master)](https://travis-ci.org/cloudyourcar/minmea)
+
 Minmea is a minimalistic GPS parser library written in pure C intended for
 resource-constrained platforms, especially microcontrollers and other embedded
 systems.