Update h5py to 2.3.1 in pip-frozen

Building h5py <= 2.3.0 fails with the latest hdf5 (1.8.13).
This was fixed in h5py 2.3.1, see https://github.com/h5py/h5py/issues/446.
New in 2.3: http://docs.h5py.org/en/2.3/whatsnew/2.3.html