doc: change build status URL (on master branch)

Author: mcolmant

README.md

@@ -3,7 +3,7 @@
 [![Join the chat at https://gitter.im/Spirals-Team/powerapi](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/Spirals-Team/powerapi?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 [![License: AGPL v3](https://img.shields.io/badge/License-AGPL%20v3-blue.svg)](http://www.gnu.org/licenses/agpl-3.0)
-[![Build Status](https://travis-ci.org/Spirals-Team/powerapi.svg)](https://travis-ci.org/Spirals-Team/powerapi)
+[![Build Status](https://travis-ci.org/Spirals-Team/powerapi.svg?branch=master)](https://travis-ci.org/Spirals-Team/powerapi)
 [![Coverage Status](https://coveralls.io/repos/Spirals-Team/powerapi/badge.svg)](https://coveralls.io/r/Spirals-Team/powerapi)
 
 PowerAPI is a middleware toolkit for building software-defined power meters.