npmInstall & bowerInstall do not run at the end

npmInstall & bowerInstall must be called in :

```
this.on('end', function() {
   
}
```

because Yeoman do not automatically schedule the task to run when everything is complete.

This behavior is only available when calling `generator.installDependencies()`.

Tested in yeoman-generator 0.17

Best regards
Merge pull request #208 from lauterry/patch-1

npmInstall & bowerInstall do not run at the end