specs: specify localtime and gmtime month values correctly.

* specs/posix_time_spec.yaml (gmtime, localtime): Add a
specification for December epoch returning month 11.
Fix description of month range to be 0-11.
* specs/posix_compat_spec.yaml (gmtime, localtime): Add a
specification for December epoch returning month 12.
Fix description of month range to be 1-12.
Fix specification to check month range 1-12.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
slingshot: sync with upstream, for git submodule fixes.

Latest slingshot bootstrap passes arguments to 'git submodule init'
correctly.
* slingshot: Sync with upstream.
* bootstrap: Update from slingshot.
* .travis.yml: Regenerate.

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