Update coverage badge

README.md

@@ -1,4 +1,4 @@
 Utility for managing mar files. MAR file format is documented at https://wiki.mozilla.org/Software_Update:MAR
 
-[![Coverage Status](https://coveralls.io/repos/mozilla/build-mar/badge.png)](https://coveralls.io/r/mozilla/build-mar)
+[![Coverage Status](https://coveralls.io/repos/mozilla/build-mar/badge.svg?branch=master&service=github)](https://coveralls.io/github/mozilla/build-mar?branch=master)
 [![Build Status](https://travis-ci.org/mozilla/build-mar.svg)](https://travis-ci.org/mozilla/build-mar)