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

Author: RalfJung

.travis.yml

@@ -33,6 +33,8 @@ before_script:
 # 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
 - export PATH=$HOME/.cargo/bin:$PATH
+- rustup default stable
+- rustup uninstall beta
 - rustup update
 # Install "master" toolchain
 - cargo install rustup-toolchain-install-master || echo "rustup-toolchain-install-master already installed"