nbconvert requires mistune

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

Test group: nbconvert E
====================================================================== ERROR:
Failure: AttributeError ('module' object has no attribute 'nbconvert')
---------------------------------------------------------------------- 
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'

---------------------------------------------------------------------- Ran 1
test in 0.001s

FAILED (errors=1