Fix failing spec. Proc.bind was removed in Rails 4.1.

Build fails with:

> /home/travis/build/nathanl/authority/spec/authority/controller_spec.rb:4:in `require': cannot load such file -- active_support/core_ext/proc (LoadError)

In issue #56 `Proc.bind` was replaced with `instance_exec`, but the controller spec still requires that `Proc` class. That file was removed in Rails 4.1 which raises an error now.