Travis CI - Rust setup

.travis.yml

language: rust
cache: cargo
rust:
  - stable
before_cache: |
  cargo install cargo-tarpaulin
before_script:
- rustup component add rustfmt
- rustup component add clippy
- cargo install cargo-audit
# As a result of https://github.com/travis-ci/travis-ci/issues/1066, we run
# everything in one large command instead of multiple commands.
# In this way, the build stops immediately if one of the commands fails.
script: |
  cargo fmt --all -- --check &&
  cargo clippy -- -D warnings &&
  cargo build &&
  cargo test &&
  cargo audit
after_success: |
  cargo tarpaulin --ignore-tests