travis: Fail build if any command in if statement fails.

Travis is checking the exit code of the entire if statement.

Fixes: 64ffc289 ("travis: add MacOS Scons build")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Acked-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
(cherry picked from commit 029b07b2)