.travis.yml: add a verbose build

I like to have a verbose build log for automation like Travis CI,
because it sometimes helps in diagnosing failures.

.travis.yml

@@ -5,6 +5,7 @@ rust:
   - nightly
 sudo: false
 script:
+  - cargo build --verbose
   - make test
   - .travis/test_features.sh
   - |