Add support for Ruby 2.1

2.1 moves the iv_index_tbl attribute into rb_classext_struct defined in
internal.h. This meant that it was no longer possible to access it
directly using RCLASS_EXT. Ruby 2.1 will cast the RCLASS object into a
custom struct, from which we can access iv_index_tbl.
Merge pull request #5 from antw/master

Add support for Ruby 2.1