Update package.json

Make things look nice on https://www.npmjs.org/package/ember-cli-spinjs and on http://addons.builtwithember.io

Currently looks like this:
![screen shot 2014-09-09 at 11 19 33 am](https://cloud.githubusercontent.com/assets/61048/4204125/b8d9cb90-3834-11e4-9ea3-0c8d6752d212.png)