Appveyor: bump rust to beta
Appveyor: bump rust to beta

Susurrus/gtk,daa84/gtk,daa84/gtk,gkoz/rust-gnome-gtk,EPashkin/rust-gnome-gtk,rust-gnome/gtk,gkoz/gtk-rs-gtk,Susurrus/gtk,rust-gnome/gtk,gkoz/gtk-rs-gtk,gkoz/rust-gnome-gtk,EPashkin/rust-gnome-gtk