Use --prefer-binary with pip

[isuruf]

--prefer-binary will use older binary wheels if the newest release doesn't have binary wheels uploaded yet.

cc @inducer

[inducer]

Thanks for the tip! I'll have to think about that; I think I almost never want wheels. Instead, I think I'd rather have the binary from conda, or one that's built right then and there. (conda-forge/pyfmmlib-feedstock#24 notwithstanding)