Update badges

Author: gkellogg

README.md

@@ -3,7 +3,7 @@
 This is an [RDF.rb][] extension that provides RDF-specific [RSpec][] matchers
 and shared examples for Ruby projects that use RDF.rb and RSpec.
 
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+[![Gem Version](https://badge.fury.io/rb/rdf-spec.svg)](https://badge.fury.io/rb/rdf-spec)
 [![Build Status](https://github.com/ruby-rdf/rdf-spec/workflows/CI/badge.svg?branch=develop)](https://github.com/ruby-rdf/rdf-spec/actions?query=workflow%3ACI)
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