CarrierWave documentation:
> Note: `recreate_versions!` will throw an exception on records without
> an image. To avoid this, scope the records to those with images or
> check if an image exists within the block.
Resolves#1976:
NoMethodError: undefined method `read' for nil:NilClass
…/carrierwave-1.3.1/lib/carrierwave/uploader/cache.rb:81:in `sanitized_file'
…/carrierwave-1.3.1/lib/carrierwave/uploader/cache.rb:118:in `cache!'
…/carrierwave-1.3.1/lib/carrierwave/uploader/versions.rb:234:in `recreate_versions!'
db/migrate/20171130172334_rebuild_conference_pictures.rb:4:in `block in up'
