travis: do not install cargo from master

Author: RalfJung

.travis.yml

@@ -35,7 +35,7 @@ before_script:
 - export PATH=$HOME/.cargo/bin:$PATH
 # Install "master" toolchain
 - cargo install rustup-toolchain-install-master || echo "rustup-toolchain-install-master already installed"
-- travis_retry rustup-toolchain-install-master -f -n master $RUSTC_HASH -c cargo -c rust-src
+- travis_retry rustup-toolchain-install-master -f -n master $RUSTC_HASH -c rust-src
 - rustup default master
 - rustc --version