Merge pull request #676 from RalfJung/appveyor

AppVeyor: ignore already existing rustup-toolchain-install-master

Author: RalfJung

.appveyor.yml

@@ -21,7 +21,7 @@ install:
     - rustup-init.exe -y --default-host %TARGET% --default-toolchain stable
     - set PATH=%USERPROFILE%\.cargo\bin;%PATH%
     # Install "master" toolchain
-    - cargo install rustup-toolchain-install-master
+    - cargo install rustup-toolchain-install-master & exit 0
     - set /p RUSTC_HASH=<rust-version
     - rustup-toolchain-install-master -f -n master %RUSTC_HASH% -c cargo -c rust-src
     - rustup default master