Rasterio wheels for Python 3.8

Rasterio wheels for Python 3.8 were less straightforward than those for Fiona, where we only had to update a git submodule. The Rasterio wheels include HDF5 and NetCDF libraries that had to be rebuilt for Python 3.8 and OS X with a newer version of XCode. I recently asked the Travis CI team for a build timeout extension and was granted it – it's more than enough to build Rasterio's dependencies for each job, now – and this came in very handy: all the Rasterio wheel-building jobs on OS X finish in less than 75 minutes. Thank you, Travis CI.

Once again, I'd like to point out that I'd be rather lost if I couldn't follow the example of https://github.com/MacPython/scipy-wheels. Thank you, Matthew Brett, et al.