Update package.json

Make things look nice on https://www.npmjs.org/package/ember-cli-proxy-fixtures and on http://addons.builtwithember.io/

Currently looks like this:
![screen shot 2014-09-09 at 11 15 13 am](https://cloud.githubusercontent.com/assets/61048/4204055/2103c5dc-3834-11e4-9615-bd4e088e424b.png)