relax release platforms check when merging distribution files

Addressing discussion in #45.

@esteve @tfoote @wjwwood Please review.
Maybe you should use the assert raises context in `nose.tools`:

```python
def foo():
    raise RuntimeError("this is a message")

with nose.tools.assert_raises_regexp(RuntimeError, "is a mess"):
    # do thing that should raise
   foo()

with nose.tools.assert_raises(RuntimeError):
    # do thing that should raise
    foo()
```