Consider no longer building 32-bit builds for conda

[mwcraig]

There is no urgency to this, but the next step we could take to reduce the appveyor run time is stop producing binary builds for 32-bit python on Windows. There are a non-negligible number of downloads for these, so if we decide to do this eventually we should provide plenty of warning.

(though with the recent change that makes pip install succeed even without compilers by using the vector.py fallback those folks could conceivably switch to pip).