travis: reduce the number of workers on OS X
travis: reduce the number of workers on OS X

It'll reduce memory usage.

cosmo0920/groonga,cosmo0920/groonga,kenhys/groonga,groonga/groonga,naoa/groonga,naoa/groonga,naoa/groonga,komainu8/groonga,komainu8/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,cosmo0920/groonga,kenhys/groonga,groonga/groonga,naoa/groonga,komainu8/groonga,cosmo0920/groonga,cosmo0920/groonga,groonga/groonga,naoa/groonga,komainu8/groonga,cosmo0920/groonga,komainu8/groonga,groonga/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,groonga/groonga,kenhys/groonga,naoa/groonga,cosmo0920/groonga,cosmo0920/groonga