Use cargo rustc instead

Overriding the RUSTFLAGS environment variable can create problems if the cargo
config sets rust flags.

.travis.yml

@@ -24,7 +24,7 @@ matrix:
 cache: cargo
 
 script:
-  - 'if [[ "$TRAVIS_RUST_VERSION" = 1.32.0 ]]; then RUSTFLAGS="-D warnings" cargo build --verbose; fi'
+  - 'if [[ "$TRAVIS_RUST_VERSION" = 1.32.0 ]]; then cargo rustc -- -D warnings; fi'
   - cargo build --verbose
   - cargo test --verbose
   - 'if [[ "$TRAVIS_RUST_VERSION" = nightly ]]; then cargo bench; fi'