The Travis VMs have 1.5 cores so we might as well make some use of the
paralellism.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Brian Jackson <iggy@theiggy.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>