Revert to caching the AR object in CacheableAttributes

Caching the attributes as JSON and manually instantiating the record
ended-up very complex since the edge-cases such as upload fields,
serialized fields, and fields with custom accessors had to be handled.

To ensure 3 points out of 4 are checked from https://gitlab.com/gitlab-org/gitlab-ce/issues/45175


we now include the Rails version in the cache key.

Signed-off-by: Rémy Coutable <remy@rymai.me>