You are using Bootstrap version 4 cdn and in version of Bootstrap 4 , it replaced img-fluid instead of img-responsive in Bootstrap 3 .
Just change the code and it will work.
<img class="img-fluid main-image" src="https://az616578.vo.msecnd.net/files/responsive/cover/main/desktop/2016/07/02/636030757259556361-1129479309_elon-musk.jpg">
And Here is the original link of Bootstrap . You can read there too .