Update ruby-activemodel to 3.0.20.

Fix CVE-2013-0333.

There is a vulnerability in the JSON  code for Ruby on Rails which
allows attackers to bypass authentication systems, inject arbitrary
SQL, inject and execute arbitrary code, or perform a DoS attack on a
Rails application.

## Rails 3.0.20 (unreleased)

* Fix XML serialization of methods that return nil to not be
  considered as YAML (GH #8853 and GH #492)


(taca)