Update to latest qless.

* Use Qless::Client#worker_name, not Qless.worker_name.
  See https://github.com/seomoz/qless/pull/156 for more info.
* Use appropriate worker class (forked or serial).
* Don't fork to see if fork is available.
  Doing so causes problems for the forking worker's
  Process.wait2 call as it thinks that is a child that died.
* Only trigger simplecov when the test process exits.