COMPAT: handle numpy 1.15 deprecation warnings
I think this is a new warning from numpy in 1.15: https://travis-ci.org/pandas-dev/pandas/jobs/409274058

```
/home/travis/build/pandas-dev/pandas/pandas/core/dtypes/common.py:2026: DeprecationWarning: elementwise == comparison failed; this will raise an error in the future.
    if dtype in [object, np.object_, 'object', 'O']:
```
