Fix for caching test failure under OSX.

Under Mac OSX ```cartopy.tests.mpl.test_caching.test_shapefile_transform_cache``` fails on my machine claiming that ```project_geometry()``` is called 0 times rather than 10. This is because the call is not being triggered by plt.draw() under the 'MacOSX' backend. Under this backend it appears that ```project_geometry()``` isn't called unless ```plt.show()``` is called.

This PR circumvents this peculiarity of the 'MacOSX' backend by running this test with the 'agg' backend.