Adjust appveyor.yml

Lookinng at this failure https://ci.appveyor.com/project/fsgit/fsharpbinding/build/1.0.224 it seems emacs is not being downloaded at the start of the AppVeyor build

I couldn't see "emacs-24.4-bin-i686-pc-mingw32.7z" but I could see "emacs-24.4-bin-i686-mingw32.7z" so trying that