Better to use -f than -e to check for a user-specified definition file. With -e, if the user accidentally types the name of a directory (or, far less likely, a device file) with the same name as the Ruby they're trying to install, they end up with ruby-build doing absolutely nothing at best, or an error message that could be pretty confusing at worst.
JRuby 9000 (the next major release) includes a backend that supports a new JVM JIT compiler called Graal. Graal is based on OpenJDK but hasn't been released yet. http://openjdk.java.net/projects/graal/