Fix LaxBoundedSemaphore' object has no attribute 'waiting'

Hi, 
I think there is a typo in the __repr__ method

I'm having this stacktrace:
[2013-11-13 14:04:22,900: ERROR/MainProcess] 'LaxBoundedSemaphore' object has no attribute 'waiting'
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/raven/utils/serializer/manager.py", line 76, in transform
    return repr(value)
  File "/usr/local/lib/python2.7/dist-packages/kombu/async/semaphore.py", line 97, in __repr__
    self.__class__.__name__, id(self), self.value, len(self.waiting),
AttributeError: 'LaxBoundedSemaphore' object has no attribute 'waiting'