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

IPython/testing/iptest.py

@@ -145,6 +145,7 b" have['sqlite3'] = test_for('sqlite3')"
 have['cython'] = test_for('Cython')
 have['tornado'] = test_for('tornado.version_info', (3,1,0), callback=None)
 have['jinja2'] = test_for('jinja2')
+have['mistune'] = test_for('mistune')
 have['requests'] = test_for('requests')
 have['sphinx'] = test_for('sphinx')
 have['jsonschema'] = test_for('jsonschema')
@@ -287,7 +288,7 b" test_sections['config'].exclude('profile')"
 
 # nbconvert:
 sec = test_sections['nbconvert']
-sec.requires('pygments', 'jinja2', 'jsonschema', 'jsonpointer')
+sec.requires('pygments', 'jinja2', 'jsonschema', 'jsonpointer', 'mistune')
 # Exclude nbconvert directories containing config files used to test.
 # Executing the config files with iptest would cause an exception.
 sec.exclude('tests.files')