Update to 2-4-stable branch

Hi,
I've created this pull request to update this gem to Spree **2-4-stable** branch and the Stripe gem to **1.16.0** version .

## List of changes:
* changed some base class from ActiveRecord::Base to Spree::Base , a child of ActiveRecord::Base that contains preferences hash itself
* added migration
* added presence test
* removed rspec-rails from Gemfile because of a warning about duplicate gem
* changes in `recurring_spec.rb` and in `spec_helper.rb` to remove deprecation warnings

Other tests are welcome!

Thank you for this gem !!