Change the "docbook" backend to refer to DocBook 5

This change updates the tests to handle the changed backend, as well as the documentation. In most of the changed tests, the output was clearly DocBook 4.5-specific (e.g. articleinfo).

This fixes issue #554. It's been tested on Ruby 1.9.3 and 2.0.0, although the changes are so trivial that I hardly think any Ruby implementation would break.

Let me know if you'd like these squashed or otherwise changed.