.travis.yml: make -j3

.travis.yml: make -j3

The move from Travis VMs to Containers came with a upgrade from 1.5
cores to 2. The received wisdom is -j N+1 means a core can be doing work
while other threads wait for IO to complete. This is hard to test on the
Travis infrastructure but an initial before/after eyeballing seems to
confirm it is an improvement.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>