Gemfile: update Rails version 3.2.15

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12241 e93f8b46-1217-0410-a6f0-8f06a7374b81
back out r12241 (back out Rails version 3.2.13)

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12242 e93f8b46-1217-0410-a6f0-8f06a7374b81