Improve warning for out-of-date dataset

This is re https://github.com/mne-tools/mne-python/issues/986

This patch works, but I'm suspecting people won't like it: I removed the `from . import tests` command from `mne.__init__`. I'm not sure why `__init__` imports every subpackage though, or when `mne.tests` would be useful in analysis code.

Anyway, this is a starting point, let me know what you think.