travis-ci: Do not make buildman warnings fatal

We currently will always see a number of warnings due to device tree
issues.  These (and other warnings) should not make the build be marked
as failure so catch exit status 129 specifically and return 0 in that
case.

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Heiko Schocher <hs@denx.de>