travis: decrease the max number of processors
travis: decrease the max number of processors

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