It seems that there are a couple of changes in Rails 3.1 so simple captcha does not work. I found a workaround though:
I changed ActionController::Streaming to ActionController::DataStreaming because the send_file method has moved in 3.1 to ActionController::DataStreaming.
The other thing i found out was that response was not set in metal controller (or there is a way to set it that i am not aware of) so i changed ActionController::Metal to ActionController::Base.
minor changes for rails 3.1 compatibility
I think my patch would not work on rails 3. I believe you should make a different branch for Rails 3.1 and keep both active.
Yep, doesn't work with rails 3: uninitialized constant ActionController::DataStreaming
uninitialized constant ActionController::DataStreaming
For rails version 3.0.* you have to put
gem 'galetahub-simple_captcha', require: "simple_captcha"
In your Gemfile