added Travis shield

Author: rafapereirabr

README.md

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 # gtfs2gps
 
+[![Travis-CI Build Status](https://travis-ci.org/ipeaGIT/gtfs2gps.svg?branch=master)](https://travis-ci.org/ipeaGIT/gtfs2gps)
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 ### Converting public transport data from GTFS format to GPS-like records
 
 **gtfs2gps** is an R package that converts public transportation data in GTFS format to GPS-like records in a `data.frame`/`data.table`, which can then be used in various applications such as running transport simulations or scenario analyses.