make sure we use the stable toolchain, no matter the cache

Author: RalfJung

.travis.yml

@@ -32,6 +32,7 @@ before_script:
   fi
 # Install Rust ("stable" toolchain for better caching, it is just used to build rustup-toolchain-install-master)
 - curl https://build.travis-ci.org/files/rustup-init.sh -sSf | sh -s -- -y --default-toolchain stable
+- rustup default stable
 - export PATH=$HOME/.cargo/bin:$PATH
 - rustup update
 # Install "master" toolchain