Removes `extend Resque::Helpers`, adds compatibility shims.

`Resque::Plugins::Status::Hash` uses `Resque::Helpers`; this PR replaces the methods it uses with compatibility shims.

I considered adding an extra test for this shimming specifically, but all the other test cases offer coverage it needs already. I can add in explicit test cases if you'd prefer.

(At the very least, this will stop the `Resque::Helpers will be gone with no replacement in Resque 2.0.0.` notice showing up in everyone's logs. See #89.)