travis: handle luadoc and ldoc specially.

Both luadoc and ldoc behave badly with lua5.2 on Travis' Ubuntu
Precise distros, but work properly if installed separately into
the system luarocks tree when called from a wrapper script.
* m4/slingshot.m4 (SS_CONFIG_TRAVIS): Generalise special Luadoc
support ldoc too, substituting GENDOC and GENDOC_FALSE into...
* travis.yml.in: ...here, where LUADOC support is also factored to
work with ldoc.
* NEWS: Update.

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