Remove build status badge for "release" branch

Author: andrie

README.md

@@ -4,8 +4,6 @@
 
 master: 
 [![Build Status](https://travis-ci.org/RevolutionAnalytics/checkpoint.svg?branch=master)](https://travis-ci.org/RevolutionAnalytics/checkpoint)
-release:
-[![Build Status](https://travis-ci.org/RevolutionAnalytics/checkpoint.svg?branch=release)](https://travis-ci.org/RevolutionAnalytics/checkpoint)
 dev: [![Build Status](https://travis-ci.org/RevolutionAnalytics/checkpoint.svg?branch=dev)](https://travis-ci.org/RevolutionAnalytics/checkpoint)
 
 **Version on CRAN**