Fix docker login docker ce 18.04-rc1+

* https://docs.docker.com/engine/reference/commandline/login/

Author: khs1994

.travis.yml

@@ -16,7 +16,7 @@ after_success:
 - sudo chmod -R 777 _book
 - echo "FROM nginx:1.13.8-alpine" >> Dockerfile
 - echo "COPY _book /usr/share/nginx/html" >> Dockerfile
-- docker login -u "$DOCKER_USERNAME" -p "$DOCKER_PASSWORD"
+- echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin
 - docker build -t dockerpracticecn/docker_practice .
 - docker run -dit --rm -p 4000:80 dockerpracticecn/docker_practice
 - sleep 5