travis: support non-travis using projects.
travis: support non-travis using projects.

* build-aux/release.mk (_travis_yml): New macro, can be set to
empty in local.mk to prevent travis rules being triggered.
(release_extra_dist): Use it instead of hard-coding.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/slingshot