slingshot: activate travis support.

* bootstrap.conf (slingshot_files): Add m4/slingshot.m4 and
travis.yml.in.
* configure.ac (SS_CONFIG_TRAVIS): Generate .travis.yml.
* .gitignore: Update.
* .travis.yml: Regenerate.

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