Add explicit size to appveyor badge

[killercup]

Appveyor badges seem to sporadically not load (for me). Previously, it looked like this:

But now, thanks to explicit width and height, it looks at least 42% less shitty:

I took the 106px width from a "build passing" badge. Let's hope the other variants are of that size, too :)

(Might also be a good idea to do that for the other badges.)

[carols10cents]

I took the 106px width from a "build passing" badge. Let's hope the other variants are of that size, too :)

It should scale if not :) Definitely looks less shitty! 🤣