Fix VS 2015 support; add to CI matrix

[yupferris]

I briefly looked into this in #78, and this was the only version (besides 2013, 2017, and 2019) that didn't work properly with the code we have today. I suspect it's likely fixable, but I don't think anybody using WS really cares about VS versions prior to the latest anyways, so I don't want to spend time on it. Still, it's nice to track an issue for it in case someone else wants to look into it.