Fix compatibility problem with Ruby 2.x

Hi,

I have fixed compatibility with Ruby 2.x (base on @Paulche [fix](https://github.com/Paulche/radiustar/blob/d3be443c0152b1f107074dcad6977cfe3a0b42a0/lib/radiustar/packet.rb#L195)).

@pjdavis Could you please merge it and publish new version? I am going to update devise-radius-authenticatable gem after that.

Thanks