Image test fails for matplotlib 1.5

[etgalloway]

This was initially reported in #29.

mplexporter's image test (mplexporter/tests/test_basic.py::test_image) fails for matlotlib 1.5 because the size of the test image has changed:

E         - of size 1220
E         ?           ^
E         + of size 1240
E         ?           ^

This is not the first time this has happened, see #22.

The test images, matplotlib 1.3.1 vs 1.5.2:

Unfortunately, this test is sensitive to relatively minor changes in how matplotlib renders images.