crash dump error on python 3.4

[berleant]

There's no issue in python 2.7 and 3.5

Discovered in this pull request: #1516
Full build details: https://travis-ci.org/nipy/nipype/builds/138532675

Traceback (most recent call last):
  File "/home/travis/miniconda3/envs/testenv/lib/python3.4/site-packages/nose/failure.py", line 39, in runTest
    raise self.exc_val.with_traceback(self.tb)
  File "/home/travis/miniconda3/envs/testenv/lib/python3.4/site-packages/nose/loader.py", line 252, in generate
    for test in g():
  File "/home/travis/build/nipy/nipype/nipype/pipeline/plugins/tests/test_base.py", line 32, in test_report_crash
    actual_crashfile = pb.report_crash(mock_node)
  File "/home/travis/build/nipy/nipype/nipype/pipeline/plugins/base.py", line 58, in report_crash
    exc_traceback)
  File "/home/travis/miniconda3/envs/testenv/lib/python3.4/traceback.py", line 181, in format_exception
    return list(_format_exception_iter(etype, value, tb, limit, chain))
  File "/home/travis/miniconda3/envs/testenv/lib/python3.4/traceback.py", line 146, in _format_exception_iter
    for value, tb in values:
  File "/home/travis/miniconda3/envs/testenv/lib/python3.4/traceback.py", line 125, in _iter_chain
    context = exc.__context__
nose.proxy.AttributeError: 'NoneType' object has no attribute '__context__'
-------------------- >> begin captured logging << --------------------
workflow: ERROR: ["Node an_id failed to run on host <MagicMock name='mock_node.result.runtime.hostname' id='140577579790464'>."]
--------------------- >> end captured logging << ---------------------