added travis-ci badge to README.md

Author: michaeljklein

README.md

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 # PandocRuby
 
+[![Build Status](https://travis-ci.org/Authorea/pandoc-ruby.svg?branch=master)](https://travis-ci.org/Authorea/pandoc-ruby)
+
 PandocRuby is a wrapper for [Pandoc](http://johnmacfarlane.net/pandoc/), a
 Haskell library with command line tools for converting one markup format to
 another.