fix appveyor being slow

[lemmatum]

[lemmatum]

@namurphy what appveyor settings do you have for plasmapy?

[lemmatum]

Based on the riot chat, perhaps we should tinker with the settings a bit. I would suggest looking into the Build and Environment settings. Check if switching to script mode improves things (as I read somewhere that build is unnecessary for Python).

[lemmatum]

This may be relevant.