I want the tests to run on a server which has no DISPLAY environment variable set, and this tells mpl to use a backend which supports this. Without it, the results are:

```
[abie@dismod mpld3]$ python test_plots/test_logscale.py >test_plots.html
Traceback (most recent call last):
  File "test_plots/test_logscale.py", line 23, in <module>
    main()
  File "test_plots/test_logscale.py", line 6, in main
    fig = plt.figure()
  File "/homes/abie/anaconda/lib/python2.7/site-packages/matplotlib/pyplot.py", line 423, in figure
    **kwargs)
  File "/homes/abie/anaconda/lib/python2.7/site-packages/matplotlib/backends/backend_qt4agg.py", line 31, in new_figure_manager
    return new_figure_manager_given_figure(num, thisFig)
  File "/homes/abie/anaconda/lib/python2.7/site-packages/matplotlib/backends/backend_qt4agg.py", line 38, in new_figure_manager_given_figure
    canvas = FigureCanvasQTAgg(figure)
  File "/homes/abie/anaconda/lib/python2.7/site-packages/matplotlib/backends/backend_qt4agg.py", line 70, in __init__
    FigureCanvasQT.__init__( self, figure )
  File "/homes/abie/anaconda/lib/python2.7/site-packages/matplotlib/backends/backend_qt4.py", line 207, in __init__
    _create_qApp()
  File "/homes/abie/anaconda/lib/python2.7/site-packages/matplotlib/backends/backend_qt4.py", line 62, in _create_qApp
    raise RuntimeError('Invalid DISPLAY variable')
RuntimeError: Invalid DISPLAY variable
```

If mpl has already been imported, `matplotlib.use('Agg')` has no effect.

There is probably a better way for me to do this... patches welcome. :)