Specify master branch for Travis build status image.

Author: junaruga

README.md

@@ -187,6 +187,6 @@ Sprockets Rails is released under the [MIT License](MIT-LICENSE).
 
 ## Code Status
 
-* [![Travis CI](https://api.travis-ci.org/rails/sprockets-rails.svg)](http://travis-ci.org/rails/sprockets-rails)
+* [![Travis CI](https://travis-ci.org/rails/sprockets-rails.svg?branch=master)](http://travis-ci.org/rails/sprockets-rails)
 * [![Gem Version](https://badge.fury.io/rb/sprockets-rails.svg)](http://badge.fury.io/rb/sprockets-rails)
 * [![Dependencies](https://gemnasium.com/rails/sprockets-rails.svg)](https://gemnasium.com/rails/sprockets-rails)