Trying to fix master build of the travis CI

This was caused because when building on master, the travis_tag variable is empty, causing the build to fail

.travis.yml

@@ -5,7 +5,11 @@ before_install:
   # see https://docs.travis-ci.com/user/environment-variables/#Defining-Variables-in-Repository-Settings
   - docker login -u="$DOCKER_USERNAME" -p="$DOCKER_PASSWORD"
 script:
-  - docker build -t "zopanix/factorio:${TRAVIS_TAG}" .
+  - |
+    if [ -z "$TRAVIS_TAG" ]; then
+      TRAVIS_TAG="latest"
+    fi
+    docker build -t "zopanix/factorio:${TRAVIS_TAG}" .
 deploy:
   provider: script
   script: docker tag "zopanix/factorio:${TRAVIS_TAG}" "zopanix/factorio:latest" && docker push "zopanix/factorio:${TRAVIS_TAG}" && docker push "zopanix/factorio:latest"