If you like, you can launch a 1.16 AMI, ssh into the instance, and run `apt-get update && apt-get install rethinkdb` to update to RethinkDB 2.1.
Thanks for being patient -- we'll get the official images updated soon.
We have an aggressive release cycle and try to release ever 2-4 months. Unfortunately the process to get an image onto AWS is fairly slow (with the exception of security patches). So the AMI images are unfortunately a little behind. We're working hard to update them as soon as possible, but unfortunately most of the process is out of our hands.