Require the jar file when requiring 'jruby-rack' itself.

Rakefile

@@ -152,6 +152,7 @@ module JRubyJars
   def self.jruby_rack_jar_path
     File.expand_path("../jruby-rack-#{JRuby::Rack::VERSION}.jar", __FILE__)
   end
+  require jruby_rack_jar_path
 end
 }
   end