ci: disable test/command/ tests on Ubuntu 16.04
ci: disable test/command/ tests on Ubuntu 16.04

Because Red Arrow 0.16.0 doesn't support Ruby 2.3 and Ubuntu 16.04
ships Ruby 2.3.

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