No particular knowledge is required, but all of our new code uses technologies like Backbone, Node, Capistrano, and MySQL.
We also make API calls into the existing system, which is written in Java, so it helps to be able to grasp a complex Java codebase with way too many layers and enterprise systems. :)
But mostly, we're just looking for great software engineers who want to make a difference, not any particular set of acronyms.
Is it the existing system the "bad" kind of Java (J2EE 1.4/EJB 1/xml hell/Abstract Factory madness) or the nicer and more modern RESTy/Dependency Injected/JEE6 (or Spring) variant?
And I recall that one of the trickier requirements is correctly sending and acking enrollment info (EDI?) to and from the many (dozens?) of insurance companies involved. Is that piece looking ok?