Update fe_bootstrap.html5

Update fe_bootstrap.html5 to use ajaxed cron.txt/php instead of old scheduler.js (just like in default fe_page.html5, see https://github.com/contao/core/commit/654babbf4e1c59d09c258cb52b1230da1e3cf6cf#diff-343d56c4683837ed52cd8c774c460818 for 3.1)