[travis-ci] add back build

Author: samcv

.travis.yml

@@ -48,7 +48,7 @@ script:
   - make test
   - export CXX=$(command -v g++-4.8 >/dev/null 2>&1; if [ $? -eq 1 ]; then printf "g++"; else printf "g++-4.8"; fi)
   - $CXX --version
-  - echo make html
+  - make html
 matrix:
   allow_failures:
     - os: osx