Update travis config to use backend builder

.travis.yml

@@ -33,7 +33,7 @@ install:
     - pip install --no-cache-dir mkdocs==1.1 mkdocs-material==4.6.3
 
 script:
-  - docker build --target=builder --tag=doccano-test .
+  - docker build --target=backend-builder --tag=doccano-test .
   - >
     if [[ "${DATABASE}" != "sqlite" ]]; then
       docker run --network doccano -e DATABASE_URL="${DATABASE_URL}" -it doccano-test sh -c 'app/manage.py wait_for_db && app/manage.py migrate && app/manage.py test api.tests server.tests'