upgrading >v4 header cells to unicode

These were causing errors downstream in nbviewer, specifically `test_url.URLTestCase`, specifically [this header](http://nbviewer.ipython.org/url/jdj.mit.edu/~stevenj/IJulia%20Preview.ipynb#Julia–Python-interoperability:-SciPy-and-Matplotlib).

I suppose the test could use a more visually recognizable unicode character than `\u2013`, but that was the example I had from the wild.