* demo.html: set js url to rawgit

Avoid MIME type checking error.

Chrome complain that:
Refused to execute script from 'https://github.com/oneyoung/jquery-calendar/raw/master/js/calendar.js' because its MIME type ('text/plain') is not executable, and strict MIME type checking is enabled. 

ref: http://stackoverflow.com/questions/17341122/link-and-execute-external-javascript-file-hosted-on-github