Pin netCDF4 to v1.2.8 everywhere

v1.2.9 honors `valid_min` and `valid_max` to mask data.  This messes up
working with data that did a sloppy job in maintaining these valid
ranges.  To learn about progress, follow:

- <https://github.com/Unidata/netcdf4-python/issues/711>
- <https://github.com/Unidata/netcdf4-python/issues/703>