_id2ref should raise RangeError rather than return nil for no object found

JRuby:

```
irb(main):001:0> ObjectSpace._id2ref 545556
=> nil
```

MRI:

```
irb(main):001:0> ObjectSpace._id2ref 545556
RangeError: 0x00000000085314 is not id value
    from (irb):1:in `_id2ref'
    from (irb):1
    from /Users/chrisseaton/.rbenv/versions/2.2.3/bin/irb:11:in `<main>'
```
