travis: regenerate .travis.yml.

* .travis.yml: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
release: be sure to remove submodules on the release branch.

* build-aux/release.mk (check-in-release-branch): Add an expression
to delete git submodules before unpacking the release tarball.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
sanity: don't complain about git submodule directories.

* build-aux/sanity.mk (_sc_excl): Add gnulib and slingshot to
exclusion list.

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