BUG/TST: Precision on downcast
See https://github.com/pydata/pandas/issues/5174

May not be just Macs. From that thread, yarikoptic's OS was: `OS: Linux 3.9-1-amd64 #1 SMP Debian 3.9.8-1 x86_64`.

A sample failure is from the generic.interpolate suite (modified to downcast, which is  the default):

``` python
    def test_interp_quad(self):
        sq = Series([1, 4, np.nan, 16], index=[1, 2, 3, 4])
        result = sq.interpolate(method='quadratic')
        expected = Series([1., 4, 9, 16], index=[1, 2, 3, 4])
        assert_series_equal(result, expected)
```

On (some?) macs and apparently some linux builds this will fail because it the result is not downcast to an `int`.  See also stepping through [here](https://github.com/pydata/pandas/issues/5174#issuecomment-26074991) and the PR to patch this failure [here](https://github.com/pydata/pandas/pull/5362).

I'm going to see if I can get my failing scipy/numpy tests to pass cleanly. Hopefully that will fix these.
