Numpy '__reduce_cython__' issue in python 3.7

[aaraney]

• Issue on python 3.7 with numpy 1.15.4, causing:

  File "include/membuf.pyx", line 16, in init netCDF4._netCDF4
AttributeError: type object 'netCDF4._netCDF4._MemBuf' has no attribute '__reduce_cython__'

• Bumping numpy requirement in setup.py > 1.15.4