add ember version badge

Author: Kelly Selden

README.md

@@ -2,6 +2,7 @@
 
 [![npm version](https://badge.fury.io/js/fastboot.svg)](https://badge.fury.io/js/fastboot)
 [![Build Status](https://travis-ci.org/ember-fastboot/fastboot.svg?branch=master)](https://travis-ci.org/ember-fastboot/ember-fastboot-server)
+![Ember Version](https://embadge.io/v1/badge.svg?start=2.3.0)
 
 FastBoot is a library for rendering Ember.js applications in Node.js.