Test serde-serialization feature on Travis CI.

Author: frewsxcv

.travis.yml

@@ -8,6 +8,7 @@ script:
   - cargo build --verbose
   - cargo test --verbose
   - cargo doc --verbose
+  - ([ $TRAVIS_RUST_VERSION != nightly ] || cargo test --verbose --features heap_size)
   - ([ $TRAVIS_RUST_VERSION != nightly ] || cargo test --features heap_size)
 
 after_success: |