Shoes cannot find its own backend

I just installed Shoes 4, but when I try the most basic program:

require 'shoes'

Shoes.app { button "test" }

It fails with this error:

NameError: uninitialized constant Shoes::Swt::App

I installed shoes using jruby -S gem install shoes --pre, like it says to on github. Why does Shoes.app give me a NameError?