module 'warnings' has no attribute 'RuntimeWarning'

[georgette-femerling]

Running moments.Spectrum.from_data_dict with default values (Polarized=True) produces the error:

   1839 if polarized:
   1840     if np.sum(fsout) == 0:
   1841         warnings.warn(
   1842             "Spectrum is empty. Did you compute the outgroup alleles "
   1843             "at variable sites?",
-> 1844             warnings.RuntimeWarning,
   1845         )
   1846     return fsout
   1847 else:

AttributeError: module 'warnings' has no attribute 'RuntimeWarning'```

Setting `polarized=False` runs.

[apragsdale]

Fixed in #180