Fix Gitbook Gitlab CI config

See https://gitlab.com/pages/gitbook/issues/8 and https://gitlab.com/pages/gitbook/merge_requests/17/diffs

.gitlab-ci.yml

@@ -1,5 +1,5 @@
 # requiring the environment of NodeJS 8.9.x LTS (carbon)
-image: node:8.9
+image: node:10
 
 # add 'node_modules' to cache for speeding up builds
 cache:
@@ -8,10 +8,8 @@ cache:
 
 before_script:
   - npm install gitbook-cli -g # install gitbook
-  - gitbook fetch latest # fetch latest stable version
+  - gitbook fetch 3.2.3 # fetch latest stable version
   - gitbook install # add any requested plugins in book.json
-  #- gitbook fetch pre # fetch latest pre-release version
-  #- gitbook fetch 2.6.7 # fetch specific version
 
 test:
   stage: test