In add: private method define_method called for JVMArgs::Rules:Class

This changes are to avoid the following error:

```
~/.rvm/gems/ruby-1.9.2-p320/gems/jvmargs-0.0.2/lib/jvmargs/rules.rb:16:in `add': private method `define_method' called for JVMArgs::Rules:Class (NoMethodError)
        from ~/.rvm/gems/ruby-1.9.2-p320/gems/jvmargs-0.0.2/lib/jvmargs/args.rb:113:in `add_rule'
        from 3.rb:7:in `<main>'
```

Works for me on Ruby 1.9.2 and Ruby 1.9.3.