relax release platforms check when merging distribution files

Addressing discussion in #45.

@esteve @tfoote @wjwwood Please review.
Fun fact, nose just refactors the `unittest.TestCase` methods into free functions: https://github.com/nose-devs/nose/blob/master/nose/tools/trivial.py#L32-54

So you could use `unittest.TestCase`, but that would require more refactoring to use their class inheritance based test layout.