BUG/TST: sorting of NaNs on sym_diff

Closes https://github.com/pydata/pandas/issues/6444, which was failing on Python 3.4

I don't actually have python 3.4 running yet, virtualenv is complaining about my 3.4 executable. These test  *should* pass on 2.6+.

I put the release note under the bug section, but I can move it to the API section if you think that's more appropriate.