protected_instance_variables is compatible with Rails < 4.1

after upgrading to `0.3.0` I was getting a `TypeError: no implicit conversion of Set into Array`.

I found [this commit](https://github.com/rails/rails/commit/c8b566d54da278a8e675115bcf2e9590f75f5eb5) that makes this change, and that is where I'm getting my < 4.1 logic.

:two_hearts: