Add npm cache clean step.

Author: icarito

.gitlab-ci.yml

@@ -24,6 +24,7 @@ pages:
   stage: docs
   image: node:8.9
   before_script:
+    - npm cache clean
     - npm install gitbook-cli -g # install gitbook
     - gitbook fetch latest # fetch latest stable version
     - gitbook install docs # add any requested plugins in book.json