fails on python 3.4

[Michael Delgado]

pandas does not seem to have a FrozenList object for python==3.4

see https://travis-ci.org/ClimateImpactLab/impactlab-tools/jobs/230154822

______________ ERROR collecting impactlab_tools/acp/aggregate.py _______________
impactlab_tools/acp/aggregate.py:3: in <module>
    from impactlab_tools.utils.cache import DataCache
impactlab_tools/utils/cache.py:3: in <module>
    import metacsv
../../../miniconda/envs/impactlab_tools/lib/python3.4/site-packages/metacsv/__init__.py:23: in <module>
    from .core import *
../../../miniconda/envs/impactlab_tools/lib/python3.4/site-packages/metacsv/core/__init__.py:5: in <module>
    from .containers import (
../../../miniconda/envs/impactlab_tools/lib/python3.4/site-packages/metacsv/core/containers.py:10: in <module>
    from pandas.core.base import FrozenList
E   ImportError: cannot import name 'FrozenList'