CLN: pyarrow 0.6.0 compat, we need to handle back-compat on this

[jreback]

/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/pyarrow/pandas_compat.py:248: FutureWarning: timestamps_to_ms=True is deprecated
warnings.warn('timestamps_to_ms=True is deprecated', FutureWarning)

[jreback]

cc @wesm

[wesm]

per discussion in apache/arrow#1046, I think the correct fix is to remove usage of timestamps_to_ms in pandas and pass the right coercion option to pyarrow.parquet.write_table