MAINT: Add ipython to requrements.txt.
MAINT: Add ipython to requrements.txt.

Just installing jupyterhub via `pip install` can't get IPython until 3.0
is released on PyPI.

See https://github.com/jupyter/jupyterhub/issues/92.

anaderi/dockerspawner,jlamcanopy/dockerspawner,Xarthisius/dockerspawner,quantopian/dockerspawner,jlamcanopy/dockerspawner,minrk/dockerspawner,betatim/dockerspawner,Fokko/dockerspawner,Fokko/dockerspawner,ibab/dockerspawner,quantopian/dockerspawner,minrk/dockerspawner,Xarthisius/dockerspawner,zonca/sagaspawner,bigzachattack/dockerspawner,bigzachattack/dockerspawner,jupyter/dockerspawner,jhamrick/dockerspawner,betatim/dockerspawner,jhamrick/dockerspawner,anaderi/dockerspawner,ibab/dockerspawner,jupyter/dockerspawner