rpm: set default centos version to 5
rpm: set default centos version to 5

On CentOS 6 and 7, centos_ver is defined. Thus, _centos_ver is correctly
defined.

On the other hand, centos_ver is not defined on CentOS 5, it means
that _centos_ver is set to 7 - this is not intended.

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