Update multibuild to a version that strips binaries in wheels

[xhochy]

On the next release, please update the multibuild submodule to matthew-brett/multibuild@b2748e5 or newer. This should reduce the size of the h5py wheels from 4.6M to 2.3M by excluding unnecessary debug symbols. The more important change is that the extracted files on disk are significantly smaller, e.g. when you install h5py in a docker container, then the size reduction is very noticeable.

[matthew-brett]

I think that's done now - can you check on the current wheels?