bootstrap: remove .travis.yml.new more aggressively.

* build-aux/bootstrap.slingshot (slingshot_update_travis_yml):
Remove .travis.yml.new unconditionally before creating a new one.
(_travis_yml_out): Correct references from this...
(_G_travis_yml_out): ...to this.
* bootstrap: Regenerate.

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