Documentation building broken in CFBot

Since recently, all Travis builds in the CFBot are broken for docs builds in
the xmllint step.  Turns out, the oasis-open.org URL to the DTD is now
redirecting to HTTPS and xmllint is normally only compiled with HTTP support
which causes this to now break.

That being said, XMLLint will load the DTD locally if available so environments
which have the packages referenced in the postgres docs won't notice as they
provide the DTD.  The package containing the DTD is however not installed on
the CFBot.  I've opened a PR against the CFBot repo [0], but until merged the
builds will be red without necessarily being an issue with the patch being
built.

cheers ./daniel

[0] https://github.com/macdice/cfbot/pull/12