Update Rainbox syntax to use the lastest presenter API

Fixes problem with old Rainbow calls:

```
'/gems/pivotal_to_pdf-formatters-0.0.4/lib/pivotal_to_pdf-formatters/default.rb:49:in `rescue in write_to': undefined method `foreground' for #<String:0x00000101f4d448> (NoMethodError)
```

Should close:
https://github.com/ywen/pivotal_to_pdf/issues/21