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>