replacing rmagick gem by mini_magick gem.

* https://github.com/carrierwaveuploader/carrierwave\#adding-versions
* https://trello.com/c/dQn0NeXp/616-rmagick-minimagick
fixing AttachmentUploader#cover filter.
fixing workflow_kit gem to version 0.0.7 until migrating to rails 4.

workflow_kit 0.0.8 only supports rails 4.