docs(README): change to the svg coveralls badge

Author: thgreasi

README.md

@@ -1,6 +1,6 @@
 # UI.Sortable directive
 [![Build Status](https://travis-ci.org/angular-ui/ui-sortable.svg)](https://travis-ci.org/angular-ui/ui-sortable)
-[![Coverage Status](https://coveralls.io/repos/angular-ui/ui-sortable/badge.png?branch=master)](https://coveralls.io/r/angular-ui/ui-sortable?branch=master)
+[![Coverage Status](https://coveralls.io/repos/angular-ui/ui-sortable/badge.svg?branch=master)](https://coveralls.io/r/angular-ui/ui-sortable?branch=master)
 [![debugInfoEnabled(false) Ready Badge](https://rawgit.com/thgreasi/ng-debugInfoDisabled-badges/master/badge1.svg)](https://docs.angularjs.org/guide/production#disabling-debug-data)
 
 This directive allows you to sort an array with drag & drop.