travis: try going back to Steve Donovan's LDoc master.

* bootstrap.conf (buildreq): Require scm-2 LDoc.
* travis.yml.in (env): Install ldoc-scm-2.rockspec.
(script): Append useful investigative output.
* .travis.yml: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>