Because Herokuapp works with HTTPS by default. You http://maxcdn.bootstrapcdn.com/ are trying to include a CDN with HTTP. This behavior is blocked by browsers by default.
Please try replacing it with the links below.
https://maxcdn.bootstrapcdn.com/bootstrap/3.3.0/css/bootstrap.min.css
https://maxcdn.bootstrapcdn.com/bootstrap/3.3.0/js/bootstrap.min.js
https://code.jquery.com/jquery-1.11.1.min.js
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