2.6-stable: Gemfile: update Rails to 3.2.20

git-svn-id: http://svn.redmine.org/redmine/branches/2.6-stable@13530 e93f8b46-1217-0410-a6f0-8f06a7374b81
Merged r13531 from trunk to 2.6-stable (#18252)

Japanese wiki_syntax_detailed.html translation updated by Go MAEDA.

git-svn-id: http://svn.redmine.org/redmine/branches/2.6-stable@13532 e93f8b46-1217-0410-a6f0-8f06a7374b81
Merge remote-tracking branch 'upstream/2.6-stable' into ro-2-6
upgrade rails-3.2.20