travis: reduce the max number of build processes
travis: reduce the max number of build processes

naoa/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga