version 0.7 only works if ADD_REVERSION_ADMIN = True

[Samuel Spencer]

From here: https://github.com/jedie/django-reversion-compare/blob/master/reversion_compare/admin.py#L225

If ADD_REVERSION_ADMIN = False or not set, Version is never imported into admin.py and Reversion compare fails on line 166

I don't really want Reversions/Versions showing up in my admin page, so it should be possible to just move the import to the top of the module.

[Jens Diemer]

Wow. Interesting. The unittests will not catch this case :(
Think there is not really a easy was to test this, isn't it?