fix coveralls badge

README.md

@@ -1,5 +1,5 @@
 [<img src="https://travis-ci.org/sul-dlss/discovery-indexer.svg?branch=master" alt="Build Status" />](https://travis-ci.org/sul-dlss/discovery-indexer)
-[<img src="https://coveralls.io/repos/sul-dlss/discovery-indexer/badge.png" alt="Coverage Status" />](https://coveralls.io/r/sul-dlss/discovery-indexer)
+[![Coverage Status](https://coveralls.io/repos/github/sul-dlss/discovery-indexer/badge.svg?branch=master)](https://coveralls.io/github/sul-dlss/discovery-indexer?branch=master)
 [<img src="https://gemnasium.com/sul-dlss/discovery-indexer.svg" alt="Dependency Status" />](https://gemnasium.com/sul-dlss/discovery-indexer)
 [<img src="https://badge.fury.io/rb/discovery-indexer.svg" alt="Gem Version" />](http://badge.fury.io/rb/discovery-indexer)