Ignore warnings when reading pickle files
Silences a warning from our tests about

```
pandas/tests/io/test_common.py::TestCommonIOCapabilities::()::test_read_fspath_all[reader10-os-/home/travis/build/pandas-dev/pandas/pandas/tests/io/data/categorical_0_14_1.pickle]
  /home/travis/build/pandas-dev/pandas/pandas/io/pickle.py:99: FutureWarning: 'pandas.core' is private. Use 'pandas.Categorical'
    return read_wrapper(lambda f: pkl.load(f))
```