Missing section in docbook backend

[florentx]

While hardening the test suite (issue #27), I discovered that one test is complaining about docbook45.conf backend:

WARNING: missing macro section: [unfloat-blockmacro]

We chose to silence this warning in the test suite for the time being, until someone contribute a patch.

[elextr]

Docbook doesn't support unfloat as far as I know, so I don't see why it can't just be an empty macro on docbook. Something like:

[unfloat-blockmacro]
#nothing to see here

should work.

[florentx]

works-for-me : https://travis-ci.org/florentx/asciidoc/builds/25250283

Thank you @elextr .
Closing in favor of PR #27