Fix Module#define_method

When defining a method through a proc which gets created out of a method
which is defined in a C-ext, `meth.ruby_method.executable` is a instance of
Rubinius::NativeCode and not Rubinius::CompiledCode which does not
respond to #scope.

This is used for example in pg gem:
https://github.com/ged/ruby-pg/blob/bb4693e811f9348f202835e701e6509d15685b0a/lib/pg/connection.rb#L187

core/module.rb

@@ -446,7 +446,11 @@ def define_method(name, meth = undefined, &prc)
     when Proc
       if meth.ruby_method
         code = Rubinius::DelegatedMethod.new(name, :call, meth, false)
-        scope = meth.ruby_method.executable.scope
+        if meth.ruby_method.executable.kind_of? Rubinius::CompiledCode
+          scope = meth.ruby_method.executable.scope
+        else
+          scope = nil
+        end
       else
         be = meth.block.dup
         be.change_name name