This is more a reminder to myself rather than a complete post because it took me a while to figure it out.
Updated the gems of AsUnit4 
as well as the sprout-flashsdk from their repository.
# rakefile.rb
##############################
# CI
# Compile the ci swf
mxmlc "bin/#{project}_ci.swf" => :asunit4 do |t|
  t.input           = "src/XMLRunner.as"
  t.source_path     << 'test'
  t.source_path     << 'src'
  t.debug           = true
end
flashplayer :cruise => "bin/#{project}_ci.swf"
`src/XMLRunner.as`
package {
    import flash.display.MovieClip;
    import asunit.core.AsUnitCore;
    import asunit.printers.XMLPrinter;
    public class XMLRunner extends MovieClip {
        public var core: AsUnitCore;
        public var printer: XMLPrinter;
        public function XMLRunner() {
            printer = new XMLPrinter();
            core = new AsUnitCore();
            core.addListener(printer);
            core.start(AllTests, null, this);
        }
    }
}