Update to Ruby2Ruby 2.0.5 and override process_defn

Brakeman was pinned to Ruby2Ruby 2.0.3 because [code introduced in 2.0.4](https://github.com/seattlerb/ruby2ruby/commit/1e001e3a2728925b998425b031a655eb104a5261#L0R338) uses `Sexp#args` (via `method_missing`) and that really conflicts with Brakeman's `Sexp#args` in a way that is not easily fixable.

However, since this behavior of Ruby2Ruby (converting methods that look like attr_* methods *into* attr_* methods) is not terribly useful for Brakeman, I've copied `process_defn` from Ruby2Ruby modulo the converting part and now we can use the latest Ruby2Ruby.