add backend through config

[mark-cooper]

• Add backend through config
• Add spec for rails backend

[mark-cooper]

This is a do not merge PR. It just gave me a chance to review this gem quickly with some cleanup.

And yes, I agree it doesn't look like we need this! If there's some merit to streamlining the cache key generation for auths/vocabs then great, that could be added to the client. But in a Rails context, just use the Rails cache directly. In other contexts, just use active support cache. This gem adds a fair bit of overhead to not do all that much and be another separate gem to maintain.

That would mean going forward, for any re-implementation of the importer, there would be no refcache dependency. All caching will be done through Rails.cache.

@kspurgin if you agree I think we can close this, plan to archive this repo at some point in the future, and perhaps create an issue for the client to do the useful part?