Add gem version svg to README

gem version badge from http://shields.io

README.md

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-[![Build Status](https://travis-ci.org/Shopify/shopify_api.svg?branch=master)](https://travis-ci.org/Shopify/shopify_api)
+[![Version][gem]][gem_url] [![Build Status](https://travis-ci.org/Shopify/shopify_api.svg?branch=master)](https://travis-ci.org/Shopify/shopify_api)
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+[gem]: https://img.shields.io/gem/v/shopify_api.svg
+[gem_url]: https://rubygems.org/gems/shopify_api
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 # Shopify API
 
 The Shopify API gem allows Ruby developers to programmatically access the admin section of Shopify stores.