Travis builds encountering weird python error

[tbohn]

Trying to run tests on my PR #793 but all Travis builds fail. The ones I've checked get the same error, which doesn't appear to be related to my code change:

================================== FAILURES===================================
_________________________________ test_strpdmy _________________________________
def test_strpdmy():
       dmy = ffi.new("dmy_struct *")
       dates = pd.date_range('2015-12-18', '2016-12-22', freq='1D')
       date_format = '%Y-%m-%d'
       for date in dates:
           date_str = date.strftime(date_format)
           vic_lib.strpdmy(date_str.encode(), date_format.encode(), dmy)
            expected = date.to_datetime()
E           AttributeError: 'Timestamp' object has no attribute 'to_datetime'
tests/unit/shared/test_vic_time.py:374: AttributeError
========== 1 failed, 55 passed, 16 skipped, 1 xpassed in 4.87 seconds ==========

[jhamman]

This is due to a change in pandas' api. I can put together a quick fix.

[jhamman]

closed via #797