to parquet does not work with default entityset

[8bit-pixies]

Bug/Feature Request Description

to_parquet does not work off entitysets consistently.

To reproduce:

import featuretools as ft
es_train = ft.demo.load_mock_customer(return_entityset=True, random_seed=0)
es_train.to_parquet("something")

Errors out with:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-13-a683aa05b604> in <module>()
----> 1 es_train.to_parquet('test')

/usr/local/anaconda/lib/python2.7/site-packages/featuretools/entityset/entityset.pyc in to_parquet(self, path)
    220         '''
    221 
--> 222         write_entityset(self, path, serialization_method='parquet')
    223         return self
    224 

/usr/local/anaconda/lib/python2.7/site-packages/featuretools/entityset/serialization.pyc in write_entityset(entityset, path, serialization_method, engine, compression)
     88                                                       metadata,
     89                                                       engine=engine,
---> 90                                                       compression=compression)
     91             elif serialization_method == 'pickle':
     92                 metadata = _write_pickle_entity_data(temp_dir,

/usr/local/anaconda/lib/python2.7/site-packages/featuretools/entityset/serialization.pyc in _write_parquet_entity_data(root, entity, metadata, engine, compression)
    168 
    169     entity_size += os.stat(df_filename).st_size
--> 170     if entity.last_time_index:
    171         rel_lti_filename = os.path.join(entity.id, 'lti.parq')
    172         lti_filename = os.path.join(root, rel_lti_filename)

/usr/local/anaconda/lib/python2.7/site-packages/pandas/core/generic.pyc in __nonzero__(self)
   1571         raise ValueError("The truth value of a {0} is ambiguous. "
   1572                          "Use a.empty, a.bool(), a.item(), a.any() or a.all()."
-> 1573                          .format(self.__class__.__name__))
   1574 
   1575     __bool__ = __nonzero__

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

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[kmax12]

@chappers thanks for the report. this is fixed in #204 and will go out in the next release