BUG: Fixed cfradial unit test to work with netCDF4 >= 1.1.1

See https://github.com/Unidata/netcdf4-python/pull/269 for change in
netCDF4 library.
Merge pull request #183 from jjhelmus/bug_netcdf_maskandscale

BUG: Fixed cfradial unit test to work with netCDF4 >= 1.1.1