COMPAT: fresh numpy 1.14 deprecation warnings
https://travis-ci.org/jreback/pandas/jobs/299274833 (this build makes the str issue in numpy 1.14 pass)
```
=============================== warnings summary ===============================
pandas/tests/test_nanops.py::TestnanopsDataFrame::()::test_nanmin
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/_methods.py:29: RuntimeWarning: invalid value encountered in reduce
    return umr_minimum(a, axis, None, out, keepdims)
pandas/tests/test_nanops.py::TestnanopsDataFrame::()::test_nanargmax
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/_methods.py:29: RuntimeWarning: invalid value encountered in reduce
    return umr_minimum(a, axis, None, out, keepdims)
pandas/tests/test_nanops.py::TestnanopsDataFrame::()::test_nanmax
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/_methods.py:26: RuntimeWarning: invalid value encountered in reduce
    return umr_maximum(a, axis, None, out, keepdims)
pandas/tests/test_nanops.py::TestnanopsDataFrame::()::test_nanargmin
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/_methods.py:29: RuntimeWarning: invalid value encountered in reduce
    return umr_minimum(a, axis, None, out, keepdims)
pandas/tests/test_resample.py::TestPeriodIndex::()::test_resample_empty_dataframe
  /home/travis/build/jreback/pandas/pandas/core/groupby.py:1917: DeprecationWarning: 0 should be passed as minlength instead of None; this will error in future.
    out = np.bincount(ids[ids != -1], minlength=ngroup or None)
  /home/travis/build/jreback/pandas/pandas/core/groupby.py:1917: DeprecationWarning: 0 should be passed as minlength instead of None; this will error in future.
    out = np.bincount(ids[ids != -1], minlength=ngroup or None)
pandas/tests/test_resample.py::TestPeriodIndex::()::test_resample_empty_series
  /home/travis/build/jreback/pandas/pandas/core/groupby.py:3477: DeprecationWarning: 0 should be passed as minlength instead of None; this will error in future.
    out = np.bincount(ids[mask], minlength=ngroups or None)
  /home/travis/build/jreback/pandas/pandas/core/groupby.py:3477: DeprecationWarning: 0 should be passed as minlength instead of None; this will error in future.
    out = np.bincount(ids[mask], minlength=ngroups or None)
  /home/travis/build/jreback/pandas/pandas/core/groupby.py:1917: DeprecationWarning: 0 should be passed as minlength instead of None; this will error in future.
    out = np.bincount(ids[ids != -1], minlength=ngroup or None)
  /home/travis/build/jreback/pandas/pandas/core/groupby.py:3477: DeprecationWarning: 0 should be passed as minlength instead of None; this will error in future.
    out = np.bincount(ids[mask], minlength=ngroups or None)
  /home/travis/build/jreback/pandas/pandas/core/groupby.py:3477: DeprecationWarning: 0 should be passed as minlength instead of None; this will error in future.
    out = np.bincount(ids[mask], minlength=ngroups or None)
  /home/travis/build/jreback/pandas/pandas/core/groupby.py:1917: DeprecationWarning: 0 should be passed as minlength instead of None; this will error in future.
    out = np.bincount(ids[ids != -1], minlength=ngroup or None)
pandas/tests/test_window.py::TestMoments::()::test_cmov_window_special_linear_range
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/scipy/signal/windows/windows.py:1489: DeprecationWarning: slepian is deprecated and will be removed in a future version, use dpss instead
    'version, use dpss instead', DeprecationWarning)
pandas/tests/test_window.py::TestMoments::()::test_cmov_window_special
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/scipy/signal/windows/windows.py:1489: DeprecationWarning: slepian is deprecated and will be removed in a future version, use dpss instead
    'version, use dpss instead', DeprecationWarning)
pandas/tests/frame/test_analytics.py::TestDataFrameAnalytics::()::test_min
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/_methods.py:29: RuntimeWarning: invalid value encountered in reduce
    return umr_minimum(a, axis, None, out, keepdims)
pandas/tests/frame/test_analytics.py::TestDataFrameAnalytics::()::test_max
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/_methods.py:26: RuntimeWarning: invalid value encountered in reduce
    return umr_maximum(a, axis, None, out, keepdims)
pandas/tests/groupby/test_functional.py::TestSize::()::test_size
  /home/travis/build/jreback/pandas/pandas/core/groupby.py:1917: DeprecationWarning: 0 should be passed as minlength instead of None; this will error in future.
    out = np.bincount(ids[ids != -1], minlength=ngroup or None)
pandas/tests/io/json/test_pandas.py::TestPandasContainer::()::test_frame_from_json_to_json
  /home/travis/build/jreback/pandas/pandas/io/json/json.py:767: DeprecationWarning: The truth value of an empty array is ambiguous. Returning False, but in future this will result in an error. Use `array.size > 0` to check that an array is not empty.
    if args:
  /home/travis/build/jreback/pandas/pandas/io/json/json.py:767: DeprecationWarning: The truth value of an empty array is ambiguous. Returning False, but in future this will result in an error. Use `array.size > 0` to check that an array is not empty.
```