Test failure: test_none_comparison
`test_none_comparison` fails for me for all versions of `numpy>=1.9.0`. I don't see this issue in any `numpy-1.8.x`  version.

```
ERROR: test_none_comparison (pandas.tests.test_base.TestIndexOps)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/ch/repo/pandas/.tox/py34/lib/python3.4/site-packages/pandas/tests/test_base.py", line 303, in test_none_comparison
    result = None != o
  File "/Users/ch/repo/pandas/.tox/py34/lib/python3.4/site-packages/pandas/core/generic.py", line 723, in __nonzero__
    .format(self.__class__.__name__))
ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().
```
