Avoid caching BroadcastMessage as an ActiveRecord object
When a Rails 4 host serializes a BroadcastMessage, it will serialize
ActiveRecord::ConnectionAdapters::PostgreSQL::OID::Integer, which does
not exist in Rails 5. This will cause Error 500s on a Rails 5 reading
from this cache.
To make Rails 4 and 5 play well together, store the data as JSON and construct the ActiveRecord objects from JSON.