Gemfile: remove "i18n"

activesupport 3.2.13 requires i18n 0.6.1 explicitly.
https://github.com/rails/rails/commit/197b9f4d61260bea9e2a0a4883977e02164a0667

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12243 e93f8b46-1217-0410-a6f0-8f06a7374b81