Show Travis build status for master branch only

Author: jcheng5

README.md

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 # An R Interface to Leaflet Maps
 
-[![Build Status](https://travis-ci.org/rstudio/leaflet.svg)](https://travis-ci.org/rstudio/leaflet)
+[![Build Status](https://travis-ci.org/rstudio/leaflet.svg?branch=master)](https://travis-ci.org/rstudio/leaflet)
 
 [Leaflet](http://leafletjs.com) is an open-source JavaScript library for
 interactive maps. This R package makes it easy to create Leaflet maps from R.