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!
```

スタックトレースを起点に cloudinary/carrier_wave.rb:160 から調査を開始し、ActiveRecord のコミット中に呼び出される delete callback を追跡します。ファイルストレージと処理を無効にしたテスト設定を再現し、enable_processing が false の場合にモデルを破棄してもリモート API 呼び出しが発生しないことを確認します。

索引モデルが issue の本文から書いたものです。