readme: added travis-ci badge

README.md

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 # freesurfer-volume-reader
 
+[![Build Status](https://travis-ci.org/fphammerle/freesurfer-volume-reader.svg?branch=master)](https://travis-ci.org/fphammerle/freesurfer-volume-reader)
+
 Python script & library to
 read hippocampal subfield volumes computed by Freesurfer