Update Travis icon to only check master branch

Author: lord

README.md

@@ -1,7 +1,7 @@
 Slate
 ========
 
-[![Build Status](https://travis-ci.org/tripit/slate.png)](https://travis-ci.org/tripit/slate) [![Dependency Status](https://gemnasium.com/tripit/slate.png)](https://gemnasium.com/tripit/slate)
+[![Build Status](https://travis-ci.org/tripit/slate.png?branch=master)](https://travis-ci.org/tripit/slate) [![Dependency Status](https://gemnasium.com/tripit/slate.png)](https://gemnasium.com/tripit/slate)
 
 Slate helps you create beautiful single-page API documentation. Think of it as an intelligent, modern documentation template for your API.