This is an example of the offending line from the Clearbook web page:
This is what we call a Content Delivery Network, rather than having all the code or images on Clearbooks own web servers, it allows us to share the load (make things quicker for the end user/you).
Web applications such as Clearbooks do not like re-inventing the wheel every time they need a feature for the website, so instead they use respected tool kits to accomplish tasks. In this example they are using a tool kit to allow them to access/request information from services. This is ok and acceptable practice.
The problem here is that AVG for many reasons have put this website domain name (cdn.rawgit.com) into a blacklist - and such you (and anyone else using AVG accessing this domain) receive the warning.
I suggest you follow the link about reporting it as a false positive, perhaps pasting the source below as being OK.
PS: I don't work for Clearbooks, so I am posting based on my understanding in hopefully plain english ;-)