configury: avoid spurious .travis.yml regeneration.

* m4/slingshot.m4 (SS_CONFIG_TRAVIS): Use cp instead of mv to
ensure the .travis.yml timestamp is updated, avoiding spurious
remakes on multiple make reruns.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
maint: move to Markdown format NEWS and release announcements.

* build-aux/do-release-commit-and-tag (noteworthy): Use a ##
prefix for new noteworthy changes lines.
* build-aux/release.mk (gl_noteworthy_news_): Update regexp.
(announcement): Generate Markdown format.
* NEWS: Reformat.
* local.mk (old_NEWS_hash): Regenerate.

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