Update for ruby 2.1.0-dev

RBASIC->klass is write protected in ruby 2.1.0-dev

Tried to use RBASIC_SET_CLASS or RBASIC_SET_CLASS_RAW but they are not available (maybe we should probably wait for now since this is not final).

Not sure what the performance impact is here.

See [Issue #42](https://github.com/brianmario/escape_utils/issues/42) for more details.