doc: use wget to download rpmforge/epel-release in CentOS 5
doc: use wget to download rpmforge/epel-release in CentOS 5

This is a workaround to avoid downloading failure in CentOS 5.

komainu8/groonga,naoa/groonga,myokoym/groonga,myokoym/groonga,myokoym/groonga,cosmo0920/groonga,kenhys/groonga,naoa/groonga,komainu8/groonga,kenhys/groonga,myokoym/groonga,groonga/groonga,groonga/groonga,myokoym/groonga,komainu8/groonga,myokoym/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,naoa/groonga,groonga/groonga,kenhys/groonga,hiroyuki-sato/groonga,redfigure/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,groonga/groonga,redfigure/groonga,groonga/groonga,redfigure/groonga,redfigure/groonga,naoa/groonga,groonga/groonga,hiroyuki-sato/groonga,kenhys/groonga,cosmo0920/groonga,naoa/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,kenhys/groonga,naoa/groonga,hiroyuki-sato/groonga,kenhys/groonga,hiroyuki-sato/groonga,kenhys/groonga,komainu8/groonga,naoa/groonga,hiroyuki-sato/groonga,komainu8/groonga,komainu8/groonga,cosmo0920/groonga,groonga/groonga,redfigure/groonga,myokoym/groonga,groonga/groonga,myokoym/groonga,naoa/groonga,cosmo0920/groonga,komainu8/groonga,redfigure/groonga,redfigure/groonga,redfigure/groonga,kenhys/groonga,komainu8/groonga,cosmo0920/groonga