Py26 str format and nosetests

For annoying reasons, I'm stuck using Python 2.6 for some projects.  This change to string formating in `mpld3/_utils.py` makes mpld3 work for me there.

The change to `process_testplots.py` makes `nosetests` run successfully, although I'm not sure you intended for nose to do the testing.

Thanks for your work on this!