cb is not a function

Using the test code provided, I'm getting the following error..

```
..node_modules/sails-html-pdf/index.js:68
                            cb(e);
                            ^
TypeError: cb is not a function
```

The only difference I have is the template dir: 
```
module.exports.pdf = {
  templateDir: 'views/pages'
}
```

It works if I kill cb at lines 65 and 68

