Have README only check TravisCI status for the master branch

[ci skip]

Author: gtback

README.md

@@ -1,5 +1,4 @@
-# python-cybox [![Build Status](https://travis-ci.org/CybOXProject/python-cybox.png)](https://travis-ci.org/CybOXProject/python-cybox)
-
+# python-cybox [![Build Status](https://travis-ci.org/CybOXProject/python-cybox.png?branch=master)](https://travis-ci.org/CybOXProject/python-cybox)
 A Python library for parsing, manipulating, and generating CybOX content.
 
 For more information about CybOX, see http://cybox.mitre.org.