No rust/cargo install in build workers (not needed anymore)

.travis.yml

@@ -27,11 +27,6 @@ before_cache:
   - rm -rf $HOME/.cache/electron-builder
 before_install:
   - rm -rf node_modules
-  # Install Rust and Cargo
-  - curl https://sh.rustup.rs -sSf > /tmp/rustup.sh
-  - sh /tmp/rustup.sh -y
-  - export PATH="$HOME/.cargo/bin:$PATH"
-  - source "$HOME/.cargo/env"
   # Install dependencies
   - travis_wait 30 yarn --frozen-lockfile