Numpy Conflict with Pandas (Compile Error)
#### Code Sample

```python
nonna = values_to_mask[-na_mask]
```
#### Problem description

In the core/common.py, there exists on line 359 the above code sample. As of now the error message is as displayed below:

TypeError: The numpy boolean negative, the `-` operator, is not supported, use the `~` operator or the logical_not function instead.

I thought this had been updated, but upon running conda update, I'm still receiving this error (I'm currently running 0.20.2 of pandas) and numpy 1.12.1 on python 2.7.

