Add a build status badge.

Author: nex3

README.md

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 A [Dart][dart] implementation of [Sass][sass]. **Sass makes CSS fun again**.
 
+[![Build Status](https://api.travis-ci.org/sass/dart-sass.svg)](https://travis-ci.org/sass/dart-sass)
+
 [dart]: https://www.dartlang.org
 [sass]: http://sass-lang.com/