Trying to install: NameError: uninitialized constant ActiveResource

When attempting to follow the guide, the following error is received.
What version of ActiveResource do I need to use this with?

Thank you,

$ rake gems:install
rake aborted!
NameError: uninitialized constant ActiveResource
/Users/albert/.rvm/gems/ruby-2.2.0/gems/fortify-0.0.3/lib/fortify/fortify.rb:1:in <top (required)>' /Users/albert/.rvm/gems/ruby-2.2.0/gems/fortify-0.0.3/lib/fortify.rb:1:in<top (required)>'
/Users/albert/.rvm/gems/ruby-2.2.0/gems/bundler-1.10.6/lib/bundler/runtime.rb:76:in require' /Users/albert/.rvm/gems/ruby-2.2.0/gems/bundler-1.10.6/lib/bundler/runtime.rb:76:inblock (2 levels) in require'
/Users/albert/.rvm/gems/ruby-2.2.0/gems/bundler-1.10.6/lib/bundler/runtime.rb:72:in each' /Users/albert/.rvm/gems/ruby-2.2.0/gems/bundler-1.10.6/lib/bundler/runtime.rb:72:inblock in require'
/Users/albert/.rvm/gems/ruby-2.2.0/gems/bundler-1.10.6/lib/bundler/runtime.rb:61:in each' /Users/albert/.rvm/gems/ruby-2.2.0/gems/bundler-1.10.6/lib/bundler/runtime.rb:61:inrequire'
/Users/albert/.rvm/gems/ruby-2.2.0/gems/bundler-1.10.6/lib/bundler.rb:134:in require' /Users/albert/Dropbox/git/hyper/config/application.rb:15:in<top (required)>'
/Users/albert/Dropbox/git/hyper/Rakefile:4:in require' /Users/albert/Dropbox/git/hyper/Rakefile:4:in<top (required)>'
/Users/albert/.rvm/gems/ruby-2.2.0/bin/ruby_executable_hooks:15:in eval' /Users/albert/.rvm/gems/ruby-2.2.0/bin/ruby_executable_hooks:15:in

'
(See full trace by running task with --trace)