Try running appveyor on master instead of nightly

appveyor.yml

@@ -15,6 +15,10 @@ install:
     - set PATH=C:\Program Files\Git\mingw64\bin;%PATH%
     - curl -sSf -o rustup-init.exe https://win.rustup.rs/
     - rustup-init.exe -y --default-host %TARGET% --default-toolchain nightly
+    - cargo install rustup-toolchain-install-master || echo "rustup-toolchain-install-master already installed"
+    - set RUSTC_HEAD=`git ls-remote https://github.com/rust-lang/rust.git HEAD | tr -d ",." | tr " \t" "\n" | grep -e "HEAD" -v`
+    - rustup-toolchain-install-master %RUSTC_HEAD%
+    - rustup default %RUSTC_HEAD%
     - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin;C:\Users\appveyor\.rustup\toolchains\nightly-%TARGET%\bin
     - if defined MSYS2_BITS set PATH=%PATH%;C:\msys64\mingw%MSYS2_BITS%\bin
     - rustc -V