Update: September 2016
Use the "Use XHR" switch placed in the main toolbar to switch between app's transport implementation and Chrome's.
You'll be required to install additional extension that behaves as a proxy between the app and Chrome but you'll be able to pass expired certificates.
Though, I still encourage you to use valid certificate in development environment. With service like letsencrypt.org it can be mostly automated and integrated into development workflow.
Original answer
You can install certificate in Chrome / System and it will work.
There is a tutorial how to do it
http://restforchrome.blogspot.co.uk/2016/04/advanced-rest-client.html
However there are plenty of tutorials explaining installation on different platforms.
Also I strongly encourage to use services like https://letsencrypt.org/ which provides free certificates. They also provide easy setup scripts for different servers.