Upgrading to Rails 4

* Changes gem versions.
* Ran rake rails:update to aid in migrating application.
Fixes Api::Controllers from Rails upgrade.

* Edits specs and controller behavior for:
  * Api::CustomersController
  * Api::PaymentsController
  * Api::PurchasesController
  * Api::RegistrationsController
  * Api::AuhtenticationController
Fixes models specs.

* Fixes specs for: Purchase, Customer, LineItem models.
Removes deprecation warnings from Rails 4 upgrade.

* Edits default_scope declaration on Transaction model.
* Edits config.secret_key_base token on initializers