addError broken with some versions of testtools

_addOutcome calls self._exc_info_to_unicode. This is not available in testtools 0.9.2.

Traceback (most recent call last):
  File "/home/jml/src/twisted/trunk/twisted/trial/test/test_reporter.py", line 1341, in test_addFailureSendsSubunitAddFailure
    self.assertForwardsToSubunit('addFailure', self.test, failure)
  File "/home/jml/src/twisted/trunk/twisted/trial/test/test_reporter.py", line 1197, in assertForwardsToSubunit
    subunitReturn = getattr(subunitClient, methodName)(*args, **kwargs)
  File "/home/jml/src/subunit/trunk/python/subunit/__init__.py", line 598, in addFailure
    self._addOutcome("failure", test, error=error, details=details)
  File "/home/jml/src/subunit/trunk/python/subunit/__init__.py", line 622, in _addOutcome
    for line in self._exc_info_to_unicode(error, test).splitlines():
exceptions.AttributeError: 'TestProtocolClient' object has no attribute '_exc_info_to_unicode'

Either the documentation should be updated, or subunit should be made more resilient to different versions of testtools.