travis: ignore CVE-2012-3386 failure on Ubuntu Precise.

Travis runs on Ubuntu 12.04 LTS, which ships an automake old
enough to still be vulnerable... but we're running CI checks
here, so that shouldn't fail the whole build!
* .travis.yml: Skip CVE-2012-3386 check.

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