Don't talk to server during tests (ActiveRecord callbacks)

This is related to https://github.com/cloudinary/cloudinary_gem/issues/23

It seems the cloudinary gem still makes remote calls to https://api.cloudinary.com/v1_1/[accountname]/image/destroy when destroy() is invoked on a model, even when CarrierWave is configured to use the file store and enable_processing is false

```
if Rails.env.test? or Rails.env.cucumber?
CarrierWave.configure do |config|
config.storage = :file
config.enable_processing = false
end
end
```

The relevant stack trace is below. We only started to notice this after upgrading to Rails 4.2 and setting `config.active_record.raise_in_transactional_callbacks = true` in application.rb. Since the remote call is made in an after_commit hook, the tests didn't see the problem.

Can you please update the cloudinary gem to skip the call is `enabled_processing==false`?

```
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/cloudinary-1.0.75/lib/cloudinary/uploader.rb:284:in `call_api'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/cloudinary-1.0.75/lib/cloudinary/uploader.rb:133:in `destroy'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/cloudinary-1.0.75/lib/cloudinary/carrier_wave.rb:160:in `delete'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/carrierwave-0.10.0/lib/carrierwave/uploader/remove.rb:15:in `block in remove!'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/carrierwave-0.10.0/lib/carrierwave/uploader/callbacks.rb:17:in `with_callbacks'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/carrierwave-0.10.0/lib/carrierwave/uploader/remove.rb:14:in `remove!'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/carrierwave-0.10.0/lib/carrierwave/mount.rb:392:in `remove!'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/carrierwave-0.10.0/lib/carrierwave/mount.rb:195:in `remove_cover_image!'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/carrierwave-0.10.0/lib/carrierwave/orm/activerecord.rb:49:in `remove_cover_image!'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:432:in `block in make_lambda'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:253:in `call'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:253:in `block in conditional'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:506:in `call'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:506:in `block in call'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:506:in `each'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:506:in `call'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:92:in `_run_callbacks'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/activesupport-4.2.1/lib/active_support/callbacks.rb:776:in `_run_commit_callbacks'
# /home/vagrant/.rvm/gems/ruby-2.1.6/gems/activerecord-4.2.1/lib/active_record/transactions.rb:314:in `committed!
```

Commencez à cloudinary/carrier_wave.rb:160 à partir de la stack trace et suivez le callback de suppression invoqué lors des commits de ActiveRecord. Reproduisez la configuration de test avec le stockage de fichiers et le traitement désactivés, puis vérifiez que la destruction d’un modèle n’effectue aucun appel à l’API distante lorsque enable_processing vaut false.

Rédigé par le modèle d'indexation à partir du texte de l'issue.