AppVeyor script outdated

[Boscop]

The AppVeyor script is outdated and uses an old rust version:
https://github.com/overdrivenpotato/rust-vst2/blob/master/appveyor_rust_install.ps1#L42
It uses rustc 1.16.0 (30cf806ef 2017-03-10) which is why the build is still failing in PR #32 due to the pub(crate) syntax which has been in stable since 1.18.

This is the current AppVeyor rust template:
https://github.com/starkat99/appveyor-rust/blob/master/appveyor.yml