Reduce rails dependency

[supremebeing7]

It would be nice if this gem could depend only on the gems from rails it needs, rather than the full rails gem. I'm not sure what specific parts are needed without digging, but I'd guess something like railties, actionview, sprockets-rails?

The whole hoopla with mimemagic versions getting yanked and rails activestorage gem depending on it really shined a light on this type of thing.

I'm happy to help in some way, even posting a PR with some changes, though I don't have a ton of time to dig through the internals of this gem.

[justin808]

@supremebeing7 Sure...PR would be great!