2.6-stable: Gemfile: use i18n 0.6 on Ruby 1.8.7 and Ruby 1.9.2

i18n 0.7 requires Ruby version >= 1.9.3

git-svn-id: http://svn.redmine.org/redmine/branches/2.6-stable@13816 e93f8b46-1217-0410-a6f0-8f06a7374b81