Fix Proc#bind being depreciated in activesupport 4.2

[dziemian007]

Currently can't use with rails 4.2 and auth_handler defined, as activesupport 4.2.0 depreciated Proc#bind method.

usecase
https://github.com/dziemian007/translator/compare/rails-4.2

Failures:

  1) Translator::TranslationsController GET index auth_handler responds 401 with auth proc defined
     Failure/Error: get :index, use_route: '/'
     NoMethodError:
       undefined method `bind' for #<Proc:0x00000004feedf0>
     # ./app/controllers/translator/translations_controller.rb:81:in `auth'
     # ./spec/controllers/translations_controller_spec.rb:27:in `block (4 levels) in <top (required)>'

[hubertlepicki]

@dziemian007 that's great fix, thanks!