Merge pull request #96 from twbs/gem-badge

Add Ruby Gem badge to README

README.md

 # [Bootstrap](http://getbootstrap.com)
 ![Bower version](https://img.shields.io/bower/v/bootstrap.svg?style=flat)
 [![npm version](https://img.shields.io/npm/v/bootstrap.svg?style=flat)](https://www.npmjs.com/package/bootstrap)
+[![Gem version](https://img.shields.io/gem/v/bootstrap.svg?style=flat)](https://rubygems.org/gems/bootstrap)
 [![Build Status](https://img.shields.io/travis/twbs/bootstrap/master.svg?style=flat)](https://travis-ci.org/twbs/bootstrap)
 [![devDependency Status](https://img.shields.io/david/dev/twbs/bootstrap.svg?style=flat)](https://david-dm.org/twbs/bootstrap#info=devDependencies)
 [![Selenium Test Status](https://saucelabs.com/browser-matrix/bootstrap.svg)](https://saucelabs.com/u/bootstrap)