Load rails_app in spec_helper and make specs run again

We have a rails_app to run tests on (under spec/), but we're not using
it yet.

* Adjust the spec_helper to start loading the rails_app via its
  environment.rb

* Remove the spec_helper's Devise::Strategies::Base, because Strategies
  is incorrectly declared as a class, and besides, we should load the
  strategy's real concrete base class to test the whole object as a unit
  (devise-3.4.0/lib/devise.rb:37:in `<module:Devise>':
  Strategies is not a module)

* Load rspec-rails in the spec_helper and set some Rails-related spec
  configs so we are ready to do rails testing

* Require 'devise' unconditionally in lib/devise_capturable.rb so we
  have it loaded in time to register the Warden strategy (line 5) and
  Devise module (line 20)

* Change the engine class name in the rails_app's config/routes.rb from
  the default to the actual name

* Add dependencies to the gemspec that are required to load and test the
  Rails app

* Fix now-failing examples by feeding an empty env hash to the strategy
  and changing attribute order in the view helper example

* Enable Capybara by requiring it in spec_helper and provide an initial
  example for demonstration purposes