add another missing rubygem mirror
add another missing rubygem mirror

cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,orange-cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,orange-cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,orange-cloudfoundry/buildpacks-ci