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

.travis.yml

@@ -7,7 +7,7 @@ services:
 
 before_install:
   - export DOCKER_REPO=nlknguyen/shellcheck
-  - export TAG=$([ "$TRAVIS_BRANCH" == "master" ] && echo "latest" || echo $TRAVIS_BRANCH)
+  - export TAG=$([ "$TRAVIS_BRANCH" == "master" ] && echo "latest" || echo $TRAVIS_BRANCH) 
 
 script:
   - docker build -t builder -f Dockerfile_builder .
@@ -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