Temporary statsmodels/scipy incompatibility

[smmaurer]

Looks like we need to pin scipy<1.3 in requirements.txt until the next release of StatsModels.

https://github.com/statsmodels/statsmodels/issues/5759

We're getting this error in all of the Python 3 Travis builds:

Traceback:
test_binary_logit.py:6: in <module>
    from urbansim_templates import modelmanager
../../../../virtualenv/python3.5.6/lib/python3.5/site-packages/urbansim_templates/modelmanager.py:12: in <module>
    from .utils import update_name, version_greater_or_equal
../../../../virtualenv/python3.5.6/lib/python3.5/site-packages/urbansim_templates/utils.py:6: in <module>
    from urbansim.models.util import (apply_filter_query, columns_in_filters,
../../../../virtualenv/python3.5.6/lib/python3.5/site-packages/urbansim/models/__init__.py:1: in <module>
    from .regression import RegressionModel, RegressionModelGroup, \
../../../../virtualenv/python3.5.6/lib/python3.5/site-packages/urbansim/models/regression.py:12: in <module>
    import statsmodels.formula.api as smf
../../../../virtualenv/python3.5.6/lib/python3.5/site-packages/statsmodels/formula/api.py:15: in <module>
    from statsmodels.discrete.discrete_model import MNLogit
../../../../virtualenv/python3.5.6/lib/python3.5/site-packages/statsmodels/discrete/discrete_model.py:45: in <module>
    from statsmodels.distributions import genpoisson_p
../../../../virtualenv/python3.5.6/lib/python3.5/site-packages/statsmodels/distributions/__init__.py:2: in <module>
    from .edgeworth import ExpandedNormal
../../../../virtualenv/python3.5.6/lib/python3.5/site-packages/statsmodels/distributions/edgeworth.py:7: in <module>
    from scipy.misc import factorial
E   ImportError: cannot import name 'factorial'

[smmaurer]

Fixed in PR #109.