Change syntax style for readability

.travis.yml

@@ -7,7 +7,7 @@ services:
 
 before_install:
   - export DOCKER_REPO=nlknguyen/shellcheck
-  - export TAG=$(if [ "$TRAVIS_BRANCH" == "master" ]; then echo "latest"; else echo $TRAVIS_BRANCH ; fi)  
+  - export TAG=$([ "$TRAVIS_BRANCH" == "master" ] && echo "latest" || echo $TRAVIS_BRANCH)
 
 script:
   - docker build -t builder -f Dockerfile_builder .
@@ -16,4 +16,5 @@ script:
 
 after_success:
   - docker login -e="$DOCKER_EMAIL" -u="$DOCKER_USERNAME" -p="$DOCKER_PASSWORD"
-  - docker push $DOCKER_REPO:$TAG
+  - [ "$TRAVIS_BRANCH" == "master" ] && docker push $DOCKER_REPO:$TAG 
+  # Only push to Docker Hub if this is the master branch