Updates yeoman-generator to ~0.13.4

This generator fails on my machine with a too-recent global `yo` package installed (`1.0.4`):

    Cannot read property 'bold' of undefined

A while back yeoman-generator switched to [chalk](https://npmjs.org/package/chalk), which has a different API for colorizing terminal text. I opted to simply remove the welcome message, since it is no longer included in the `generator-generator` template.

The #prompt callback signature has also changed, so I updated the call to that method as well.

This module now works & mocha tests now pass with a modern yeoman installation.