Update CI config

.gitlab-ci.yml

 image: node:10
 
 before_script:
+  - apt --yes update
+  - apt --yes install --no-install-recommends rsync
+  # image node:10 already provides openssh-client
+
   - npm install gitbook-cli -g
 
-  # image node:10 already provides openssh-client
 
   # Run ssh-agent (inside the build environment).
   - eval $(ssh-agent -s)