Upgrade redmine installation?

Since the recent critical security issues with Rails and the subsequent releases to fix them, how can I most easily upgrade my redmine installation to avoid it becoming compromised?

As I understand it, I only need to upgrade rails. But that doesn't seem to be straightforward:

[cgk@web186 redmine]$ which gem
~/webapps/redmine/bin/gem
[cgk@web186 redmine]$ gem update rails
Updating installed gems
Nothing to update
[cgk@web186 redmine]$ gem list

*** LOCAL GEMS ***

bundler (1.2.3)
daemon_controller (0.2.6)
edavis10-object_daddy (0.4.3)
fastthread (1.0.7)
i18n (0.4.2)
metaclass (0.0.1)
mocha (0.10.3)
mysql (2.8.1)
passenger (3.0.9)
rack (1.3.2, 1.1.0)
rake (10.0.3, 0.9.2)
rdoc (2.4.2)
rubygems-update (1.8.24, 1.5.3)
shoulda (2.10.3)
sqlite3 (1.3.5)
sqlite3-ruby (1.3.3)
svn2git (2.1.2)

I'm relatively sure that I installed redmine by using webfaction's automated installer, and it's a Rails app, but where does Rails come from? Is it using some system-wide Rails install which has already been patched? (I can't tell because there's no rails command in my path)

If it is using some local and hence still vulnerable version of Rails, how do I go about upgrading it?