Skip tests which need override_settings(MEDIA_ROOT) in Django 1.4.

override_settings(MEDIA_ROOT) isn't made to sort out FileSystemStorage
until Django 1.5: (https://github.com/django/django/commit/a24ffa52d029)
so let's skip tests that need that in 1.4.

In speaker_tests, instead of skipping the whole test, which does other
useful stuff, I've just put an if around an assertion.