Appveyor: bump Rust version and update mingw
Appveyor: bump Rust version and update mingw

hmeyer/gdk,gkoz/rust-gnome-gdk,hmeyer/gdk,gkoz/gtk-rs-gdk,rust-gnome/gdk,gkoz/rust-gnome-gdk,rust-gnome/gdk,gkoz/gtk-rs-gdk