Replace lena with astronaut

This is based on #1126 
From the EuroSciPy sprint I seem to recall certain tests passing on Lena but not if substituted for other images. I believe generally speaking what happened here was improvements to the tests to make them require a close tolerance rather than exact equality, which was necessary to pin down stubborn Travis failures after the substitution.

Also I believe `assert_allclose` is recommended for use over `assert_array_almost_equal`.

If any of that is wrong, please correct me @stefanv or @rebeccaroisin!