show Travis status only for master branch [ci skip]

Author: Francesco Rodriguez

README.md

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 # Official Ruby-on-Rails Integration with Sass
-[![Travis CI](https://secure.travis-ci.org/rails/sass-rails.png)](http://travis-ci.org/rails/sass-rails) [![Gem Version](https://badge.fury.io/rb/sass-rails.png)](http://badge.fury.io/rb/sass-rails)
+[![Travis CI](https://secure.travis-ci.org/rails/sass-rails.png?branch=master)](http://travis-ci.org/rails/sass-rails) [![Gem Version](https://badge.fury.io/rb/sass-rails.png)](http://badge.fury.io/rb/sass-rails)
 
 This gem provides official integration for Ruby on Rails projects with the Sass stylesheet language.