Improve maintenance of gem

[morgoth]

Currently delayed_paperclip supports lot of background backends. It would be easier to maintain code if we could reduce it.

My idea is to move forward and go altogether with paperclip 5.0 (which currently is in beta) and drop support for Rails < 4.2 and Ruby 2.0 https://github.com/thoughtbot/paperclip/blob/f7aa32069cc798dc96ca07c8dceee2ced7a1b571/NEWS#L5-L6

Rails 4.2 introduced ActiveJob, so what do you think about dropping support for other backends and use only AJ?

[ScotterC]

@morgoth I think we should do this with a major version change so that we can keep an existing old version of DP, unmaintained, for anyone that wants to make use of the old backends

[morgoth]

Yes, sure - with some deprecation warning in minor release.
Altogether with #169 what should be the next release? Major or just minor?

[morgoth]

ongoing on #178