travis: move .travis.yml regeneration from configure to bootstrap.

* m4/slingshot.m4: Remove unused file.
* configure.ac (SS_CONFIG_TRAVIS): Remove.
* bootstrap.conf (travis_extra_rocks): New variable to list rocks
formerly listed in SS_CONFIG_TRAVIS parameters.
* build-aux/bootstrap.slingshot (slingshot_update_travis_yml): New
function to ensure .travis.yml doesn't go out of date.
* bootstrap: Regenerate.

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