Fix a spec on Kernel#remove_instance_variable for 2.0

Looks already commited to rubyspec by Shugo Maeda. Thanks!
https://github.com/rubyspec/rubyspec/commit/4c779f799551b506331f3bd198e15517f5e31b8c
Reorder Kernel by Array to use public in Kernel definition

https://github.com/rubinius/rubinius/pull/2484#pullrequestreviewcomment-5313637
Fix Kernel#remove_instance_variable for 2.0
Merge pull request #2484 from kachick/2.0-kernel-rm_ivar

Kernel#remove_instance_variable should be a public method from 2.0