irkerd: Replace Exception.format_exc() with traceback.format_exc()
irkerd: Replace Exception.format_exc() with traceback.format_exc()

The former was giving me:

  Traceback (most recent call last):
    File "/usr/lib64/python3.3/threading.py", line 901, in _bootstrap_inner
      self.run()
    File "/usr/lib64/python3.3/threading.py", line 858, in run
      self._target(*self._args, **self._kwargs)
    File "./irkerd", line 637, in dequeue
      LOG.debug(e.format_exc())
  AttributeError: 'TypeError' object has no attribute 'format_exc'

In Python 3.3.4.

dak180/irker,Southen/irker