Use RBASIC_SET_CLASS macro to build under Ruby 2.1.0.

This change uses RBASIC_SET_CLASS(obj, val) in Ruby 2.1.0, not RBASIC(obj)->klass = val, which is used in other version. 

This uses Ruby internal macros brought in with GC changes in 2.1.0, if there is a better way to achieve this, I'm all ears.