travis-ci: run Coccinelle static analysis with two parallel jobs

travis-ci: run Coccinelle static analysis with two parallel jobs

Currently the static analysis build job runs Coccinelle using a single
'make' job.  Using two parallel jobs cuts down the build job's run
time from around 10-12mins to 6-7mins, sometimes even under 6mins
(there is quite large variation between build job runtimes).  More
than two parallel jobs don't seem to bring further runtime benefits.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>