New cftime 1.5.0 breaks nc-time-axis

[aulemahal]

Hi!

Looks like cftime 1.5.0 breaks nc-time-axis... I think it is due to Unidata/cftime#235, which removed cftime.utime...

With my use case, I get (end part of the traceback):

 ~/work/xclim/xclim/.tox/doctests/lib/python3.7/site-packages/nc_time_axis/__init__.py in convert(cls, value, unit, axis)
    286                                  '`cftime.datetime`.')
    287 
--> 288         ut = cftime.utime(cls.standard_unit, calendar=first_value.calendar)
    289 
    290         if isinstance(value, (CalendarDateTime, cftime.datetime)):

AttributeError: module 'cftime' has no attribute 'utime'

[bjlittle]

Closed by #66