Fix for Rubinius 2.0's gem-based toolset

Rubinius 2.0 introduced a new system for the AST, compiler, and generator parts of their toolchain. Instead of being part of the main Rubinius repo and standard library, these are now in their own separate gems. This probably isn't the best way for getting everything updated, but hopefully it can serve as a jumping off point for breakages (since it builds fine with rbx-2.0.0 and these changes (with a `bundle install` before building)).