Build status badge in README refers to master branch.

Otherwise, experimental branches being pushed can cause the badge
in the README (master branch, GitHub front page) to show the build
failing.

README.markdown

@@ -2,7 +2,7 @@
 
 [![Gem Version](https://badge.fury.io/rb/jekyll.png)](http://badge.fury.io/rb/jekyll)
 
-[![Build Status](https://secure.travis-ci.org/mojombo/jekyll.png)](https://travis-ci.org/mojombo/jekyll)
+[![Build Status](https://secure.travis-ci.org/mojombo/jekyll.png?branch=master)](https://travis-ci.org/mojombo/jekyll)
 [![Code Climate](https://codeclimate.com/github/mojombo/jekyll.png)](https://codeclimate.com/github/mojombo/jekyll)
 [![Dependency Status](https://gemnasium.com/mojombo/jekyll.png)](https://gemnasium.com/mojombo/jekyll)
 [![Coverage Status](https://coveralls.io/repos/mojombo/jekyll/badge.png)](https://coveralls.io/r/mojombo/jekyll)