Configure travis to update gitbook in gh-pages branch

[teles]

🎉 Github pages for FrontPress documentation is already working.

But there is a problem. I made the gh-pages manually by myself. Would be nice to configure travis to build the gh-pages. Travis script should be something like this:

• Make sure travis install gitbook-cli

rm .gitignore
gitbook serve
mv _book/* .
git add -A .
git commit -m "Update gh-pages"
git push --force --quiet "https://${GH_TOKEN}@${GH_REF}" master:gh-pages > /dev/null 2>&1 
# above command got from here https://gist.github.com/domenic/ec8b0fc8ab45f39403dd

This script will run after any new master update.

Nice to have: @frontpressbot could be the author from these build commits.