As Google moved the source over to GitHub a while back, the new GitHub version can be accessed from RawGit by using the following script url:
https://cdn.rawgit.com/googlemaps/js-marker-clusterer/gh-pages/src/markerclusterer.js
You'll also need to specify the imagePath option when instantiating your MarkerClusterer to access the images from GitHub:
var mc = new MarkerClusterer(map, markers, {
imagePath: 'https://cdn.rawgit.com/googlemaps/js-marker-clusterer/gh-pages/images/m'
});
The following earlier SO post contains more detail regarding the imagePath reference to the cluster images:
Google maps not working in https://
Whilst the above urls (with the cdn prefixes) have no traffic limits or throttling and the files are served via a super fast global CDN, please bear in mind that RawGit is a free hosting service and offers no uptime or support guarantees.
This is covered in more detail in the following SO answer:
Link and execute external JavaScript file hosted on GitHub
This post also covers that, if you're linking to files on GitHub, in production you should consider targeting a specific release tag to ensure you're getting a specific release version of the script.
However, as the custodians of the js-marker-clusterer repository have yet to create any releases, this isn't currently possible.
As a result, you should seriously consider downloading and including the library and its resources directly in your project for production purposes.