Updated Gruntfile to Grunt 0.4 API

I was playing with your plugin and ended up [upgrading the Gruntfile](http://gruntjs.com/upgrading-from-0.3-to-0.4) to work with Grunt 0.4.  Nice plugin :)

**grunt release** runs with no errors:

![grunt-release](https://f.cloud.github.com/assets/451178/2147373/5036ca5a-93d1-11e3-8b27-091ff3baf759.png)