travis: Correct error checking when building boards

At present if buildman reports an error, the travis build still succeeds.

This is because the travis script does not stop when it sees errors; nor
does it automatically return the exit code. Also the current error
checking never triggers since 'ret' is not set.

Fix this by setting 'ret' correctly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>