Try running appveyor on master instead of nightly

Author: oli-obk

appveyor.yml

@@ -17,6 +17,10 @@ install:
     - rustup-init.exe -y --default-host %TARGET% --default-toolchain nightly
     - 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
+    - cargo install rustup-toolchain-install-master || echo "rustup-toolchain-install-master already installed"
+    - rustup-toolchain-install-master -f -n master
+    - rustup default master
+    - set PATH=%PATH%;C:\Users\appveyor\.rustup\toolchains\master\bin
     - rustc -V
     - cargo -V