Run tests in --release mode in travis too

Author: bluss

.travis.yml

@@ -27,4 +27,5 @@ script:
       cargo test --verbose &&
       ([ -z "$FEATURES" ] || cargo build --verbose --features "$FEATURES") &&
       ([ -z "$FEATURES" ] || cargo test --verbose --features "$FEATURES") &&
+      ([ -z "$FEATURES" ] || cargo test --release --verbose --features "$FEATURES") &&
       ([ "$BENCH" != 1 ] || cargo bench --no-run --verbose --features "$FEATURES")