there is a potential bug with respect to matplotlib2 in tick locations

the version of mpl is now pinned, pending this fix

https://github.com/SciTools/nc-time-axis/blob/master/nc_time_axis/tests/unit/test_NetCDFTimeDateLocator.py#L99
https://github.com/SciTools/nc-time-axis/blob/master/nc_time_axis/tests/unit/test_NetCDFTimeDateLocator.py#L107
https://github.com/SciTools/nc-time-axis/blob/master/nc_time_axis/tests/unit/test_NetCDFTimeDateLocator.py#L111

ref #22