fix travis

Author: vangie

.travis.yml

@@ -36,7 +36,7 @@ script:
 - cd ../frontend-webjars && mvn install -DskipTests=true -Dmaven.javadoc.skip=true -B -V
 - cd ../backend && mvn package -DskipTests=true -Dmaven.javadoc.skip=true -B -V
 - cd .. && docker build -f Dockerfile -t $REPO:$COMMIT .
-- if [ -n $TRAVIS_TAG ]; then docker tag $REPO:$COMMIT $REPO:$TRAVIS_TAG ; docker tag $REPO:$COMMIT $REPO:latest ; docker login -u $DOCKER_USER -p $DOCKER_PASS ; docker push $REPO ; fi
+- if [ -n "$TRAVIS_TAG" ]; then docker tag $REPO:$COMMIT $REPO:$TRAVIS_TAG ; docker tag $REPO:$COMMIT $REPO:latest ; docker login -u $DOCKER_USER -p $DOCKER_PASS ; docker push $REPO ; fi
 
 before_cache:
 - set -e