Correct quoting

Author: orf

.travis.yml

@@ -101,7 +101,7 @@ env:
 - COMPOSE_APP=mariadb PYTHON_VERSION=3.7 MARIADB_VERSION=10.3
 
 before_install:
-  - echo DOCKER_OPTS="${DOCKER_OPTS} --registry-mirror=https://mirror.gcr.io" | sudo tee -a /etc/default/docker
+  - echo 'DOCKER_OPTS="${DOCKER_OPTS} --registry-mirror=https://mirror.gcr.io"' | sudo tee -a /etc/default/docker
   - sudo service docker restart
 
 install: