Use grunt option with grunt.util.spawn

Detecting the `grunt` binary can be tricky as grunt can exists in many different ways. We found using `process.argv[0]` and `process.argv[1]` to be the most reliable so in Grunt v0.4 we added a `grunt: true` option to `grunt.util.spawn`.

This change will ensure spawning `grunt` will work with more environments and setups. Reference [this stackoverflow question](http://stackoverflow.com/questions/16131084/using-grunt-with-non-global-installation).

Thanks!