Update Gem Version image.

Author: gkellogg

README.md

@@ -5,7 +5,7 @@ This is a pure-Ruby library for working with [Resource Description Framework
 
 * <https://ruby-rdf.github.io/rdf>
 
-[![Gem Version](https://badge.fury.io/rb/rdf.png)](https://badge.fury.io/rb/rdf)
+[![Gem Version](https://badge.fury.io/rb/rdf.svg)](https://badge.fury.io/rb/rdf)
 [![Build Status](https://github.com/ruby-rdf/rdf/workflows/CI/badge.svg?branch=develop)](https://github.com/ruby-rdf/rdf/actions?query=workflow%3ACI)
 [![Coverage Status](https://coveralls.io/repos/ruby-rdf/rdf/badge.svg?branch=develop)](https://coveralls.io/github/ruby-rdf/rdf?branch=develop)
 [![Gitter chat](https://badges.gitter.im/ruby-rdf/rdf.png)](https://gitter.im/ruby-rdf/rdf)