Do not retrieve disabled versions

Fixes https://github.com/carrierwaveuploader/carrierwave/issues/1350 (copy from it's content below)

The code speaks for itself

```ruby
require 'carrierwave'

class BugUploader < CarrierWave::Uploader::Base
  version :thumb, if: :always_false do
    def default_url
      'default.png'
    end
  end

  def root
    '~'
  end

  def always_false(*args)
    false
  end
end

up1 = BugUploader.new
up1.store!(File.open(__FILE__))
puts up1.thumb.url
# default.png

up2 = BugUploader.new
up2.retrieve_from_store!('lalala.png')
puts up2.thumb.url
# /uploads/thumb_lalala.png

```