readme: point travis badge to explicitly to master branch

Author: darwin

readme.md

@@ -2,7 +2,7 @@
 
 [![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg)](license.txt) 
 [![Clojars Project](https://img.shields.io/clojars/v/binaryage/dirac.svg)](https://clojars.org/binaryage/dirac) 
-[![Travis](https://img.shields.io/travis/binaryage/dirac.svg)](https://travis-ci.org/binaryage/dirac) 
+[![Travis](https://img.shields.io/travis/binaryage/dirac/master.svg)](https://travis-ci.org/binaryage/dirac) 
 [![Sample Project](https://img.shields.io/badge/project-example-ff69b4.svg)](https://github.com/binaryage/dirac-sample) 
 [![Chrome Extension](https://img.shields.io/badge/chrome-extension-ebb338.svg)](https://chrome.google.com/webstore/detail/dirac-devtools/kbkdngfljkchidcjpnfcgcokkbhlkogi)