Add bootbox.js 4.0.0 library

Source taken from https://github.com/makeusabrew/bootbox/releases/download/v4.0.0/bootbox.min.js

Main repository: https://github.com/makeusabrew/bootbox

I'm the author of Bootbox and have received a request to make the library available on cdnjs; I believe I've followed all the instructions correctly but please do just let me know if not :).