Fix building documentation

.travis.yml

@@ -77,6 +77,7 @@ jobs:
         - sudo sh -c "echo ttf-mscorefonts-installer msttcorefonts/accepted-mscorefonts-eula select true | debconf-set-selections"
         - sudo apt update && sudo apt install -y calibre msttcorefonts
         - npm install gitbook-cli -g
+        - gitbook fetch 3.2.3 && npm i npm@3.10.10 --prefix=~/.gitbook/versions/3.2.3/ # fixing https://travis-ci.org/github/CopterExpress/clover/jobs/766541125#L932
         - npm install markdownlint-cli -g
         - npm install svgexport -g
         - gitbook -V