Fix syntax error with traditional if-clause instead of escaping bracket

.travis.yml

@@ -16,5 +16,6 @@ script:
 
 after_success:
   - docker login -e="$DOCKER_EMAIL" -u="$DOCKER_USERNAME" -p="$DOCKER_PASSWORD"
-  - [ "$TRAVIS_BRANCH" == "master" ] && docker push $DOCKER_REPO:$TAG 
-  # Only push to Docker Hub if this is the master branch 
+  - if [ "$TRAVIS_BRANCH" == "master" ]; then \
+      docker push $DOCKER_REPO:$TAG           \
+    fi