Update Rails to 3.2.19

I see there is some ongoing work to upgrade to [Rails 4](https://github.com/loomio/loomio/pull/1567), which is awesome! But I think it's wise to update to the latest security release of Rails 3.2 in the meantime.

I ran RSpec and Cucumber locally and it's all green :)

Link to the CVEs: https://groups.google.com/forum/#!msg/rubyonrails-security/wDxePLJGZdI/WP7EasCJTA4J