fix appveyor install instructions

[fdopen]

Updating gcc is not longer necessary, because appveyor updated their images and a recent enough version of gcc is now installed by default. The current instructions don't work any longer, because the most recent version of mingw64-i686-gcc-core would also require a newer versions of mingw64-x86_64-binutils.

This hopefully fixes #325

The second commit is not intended for merge. I will remove it, once the CI is happy.

[fdopen]

Second commit removed. It will now fail again, because appveyor.yml points to this repo again instead of my fork.

previous successful run: https://ci.appveyor.com/project/samoht/ocaml-ci-scripts/builds/31503599

successfull runs with another project: https://ci.appveyor.com/project/fdopen/ocaml-ctypes/builds/31503070 https://ci.appveyor.com/project/fdopen/ocaml-ctypes/builds/31503949

[samoht]

Many thanks!