Display the latest released gem version.

The reason I used an HTML img tag here is to support Retina 
resolution wherever possible.

This is something we're working on for all Shields metadata badges in general but Michael from Gemfury already supports it:
http://badge.fury.io/for/rb/bundler

README.md

@@ -1,5 +1,5 @@
 [![Code Climate](https://codeclimate.com/github/bundler/bundler.png)](https://codeclimate.com/github/bundler/bundler)
-[![Build Status](https://secure.travis-ci.org/bundler/bundler.png?branch=1-3-stable)](http://travis-ci.org/bundler/bundler)
+[![Build Status](https://secure.travis-ci.org/bundler/bundler.png?branch=1-3-stable)](http://travis-ci.org/bundler/bundler) <a href="http://badge.fury.io/rb/bundler"><img src="https://badge.fury.io/rb/bundler@2x.png" alt="Gem Version" height="18"></a>
 
 # Bundler: a gem to bundle gems
 Bundler keeps ruby applications running the same code on every machine.