nbconvert requires mistune

You get a cryptic error message from iptest if mistune is not installed.

Test group: nbconvert
E
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ERROR: Failure: AttributeError ('module' object has no attribute 'nbconvert')
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Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/nose/loader.py", line 403, in loadTestsFromName
    module = resolve_name(addr.module)
  File "/usr/lib/python2.7/dist-packages/nose/util.py", line 321, in resolve_name
    obj = getattr(obj, part)
AttributeError: 'module' object has no attribute 'nbconvert'

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Ran 1 test in 0.001s

FAILED (errors=1
Merge pull request #6280 from ivanov/mistune

nbconvert requires mistune