unifont.otf doesn't work

Text written in `unifont.otf` does not show up.  There is no error, it just doesn't display anything.  Other fonts work fine.  I have Prawn 2.5.0.

```ruby
require 'prawn'

Prawn::Document.generate('font_test.pdf') do
  font('unifont.otf') {text 'Hello World'}
end
```