configury: simplify regeneration rules for .travis.yml.

When called by make as just `config.status .travis.yml`, the
travis-yml-up-to-date commands did not run, and so a spurious
.travis.yml.new file dropping would appear occasionally.
* m4/slingshot.m4 (SS_CONFIG_TRAVIS): Consolidate regeneration
log entirely in the AC_CONFIG_FILES call for .travis.yml to
prevent this, and simplify accordingly.

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