require at least ipython 6.1 so we can use _repr_mimebundle

Author: jasongrout

setup.py

@@ -111,6 +111,7 @@
 setuptools_args = {}
 install_requires = setuptools_args['install_requires'] = [
     'ipykernel>=4.5.1',
+    'ipython>=6.1.0', # to use _repr_mimebundle
     'traitlets>=4.3.1',
     # Requiring nbformat to specify bugfix version which is not required by
     # notebook.