Make sure we show build status for master (#44)

Author: Nyholm

Readme.md

@@ -1,7 +1,7 @@
 # Translation extractor
 
 [![Latest Version](https://img.shields.io/github/release/php-translation/extractor.svg?style=flat-square)](https://github.com/php-translation/extractor/releases)
-[![Build Status](https://img.shields.io/travis/php-translation/extractor.svg?style=flat-square)](https://travis-ci.org/php-translation/extractor)
+[![Build Status](https://travis-ci.org/php-translation/extractor.svg?branch=master)](http://travis-ci.org/php-translation/extractor)
 [![Code Coverage](https://img.shields.io/scrutinizer/coverage/g/php-translation/extractor.svg?style=flat-square)](https://scrutinizer-ci.com/g/php-translation/extractor)
 [![Quality Score](https://img.shields.io/scrutinizer/g/php-translation/extractor.svg?style=flat-square)](https://scrutinizer-ci.com/g/php-translation/extractor)
 [![SensioLabsInsight](https://insight.sensiolabs.com/projects/a5ae0cbe-f213-4ba6-94da-e9cffed86256/mini.png)](https://insight.sensiolabs.com/projects/a5ae0cbe-f213-4ba6-94da-e9cffed86256)