Fix matplotlib tests on Travis

Matplotlib doesn't work out of the box on Travis because there is no `$DISPLAY` variable (well, there's no graphics in general) and the default backend is `QtAgg`. This PR is to get that working so the unittests can cover the plotting functions too.

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What eventually worked:

I added a matplotlibrc file under `testing/` with only the line

    backend : Agg

And then as part of the `install` section of the Travis config I copy this into the working directory so it gets picked up by the `nosetests` process.