README: Use SVG badges

Author: olleolleolle

README.md

@@ -5,7 +5,7 @@ Rails integration for [jsonapi-rb](http://jsonapi-rb.org).
 
 [![Gem Version](https://badge.fury.io/rb/jsonapi-rails.svg)](https://badge.fury.io/rb/jsonapi-rails)
 [![Build Status](https://secure.travis-ci.org/jsonapi-rb/jsonapi-rails.svg?branch=master)](http://travis-ci.org/jsonapi-rb/jsonapi-rails?branch=master)
-[![Gitter chat](https://badges.gitter.im/gitterHQ/gitter.png)](https://gitter.im/jsonapi-rb/Lobby)
+[![Gitter chat](https://badges.gitter.im/gitterHQ/gitter.svg)](https://gitter.im/jsonapi-rb/Lobby)
 
 ## Resources