Fix clippy travis setting

Author: h-michael

.travis.yml

@@ -8,7 +8,7 @@ before_script:
   - if [ "$TRAVIS_RUST_VERSION" = "stable" ]; then (rustup component add rustfmt-preview clippy-preview) fi
 script:
   - if [ "$TRAVIS_RUST_VERSION" = "stable" ]; then (cargo fmt --all -- --check) fi
-  - if [ "$TRAVIS_RUST_VERSION" = "stable" ]; then (cargo clippy) fi
+  - if [ "$TRAVIS_RUST_VERSION" = "stable" ]; then (cargo clippy -- -D warnings) fi
   - cargo test --all
   - cargo build --manifest-path=./graphql_client/examples/github/Cargo.toml
   - cargo build --manifest-path=./graphql_client_cli/Cargo.toml