travis: consolidate special treatment for luadoc and specl.

* m4/slingshot.m4 (SS_CONFIG_TRAVIS): Set up substitutions keyed
on whether 'luadoc' or 'specl' appear in the argument list.
* configure.ac (SS_CONFIG_TRAVIS): Add specl.
* travis.yml.in: Set up a subsitution to comment or uncomment the
appropriate special handling of luadoc and specl.
* .travis.yml: Regenerate.

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