Removes download button

Replaces it with `npm install -g ember-cli`

![image](https://cloud.githubusercontent.com/assets/1131196/2626573/850e3598-bdd8-11e3-95a4-7a4cf9514127.png)

![ember cli ember command line utility for the ambitions web applications 2014-04-06 18-12-41 2014-04-06 18-12-46](https://cloud.githubusercontent.com/assets/1131196/2626577/96651c30-bdd8-11e3-9557-78d44bd8b3fc.jpg)