feat(readme): dependencies badge

If you [login to Gemnasium](https://gemnasium.com/login), and [add a project](https://gemnasium.com/projects/new_from_github), we can have a nice badge and a web visual to all dependencies, which would be nice, in my opinion.

It would look something like this:
![up-to-date](http://img.shields.io/badge/dependencies-up--to--date-brightgreen.svg)
![out](http://img.shields.io/badge/dependencies-out--of--date-yellow.svg)
![update](http://img.shields.io/badge/dependencies-update!-red.svg)