Don't depend on `Rails` for Redis configuration file paths
The Rails object was not always available in all tasks that require
Redis access, such as mail_room, so the constant pointing to the
configuration path was never defined, but we still attempted to access
it in config_file_name, resulting in a NameError exception.
Further, there was no benefit to defining these paths in a constant to begin with -- they're only accessed in one place, and it was within the class where they were being defined. We can just provide them at run-time instead.
Further still, we were calling File.expand_path on the absolute path
returned by Rails.root.join, which was rather pointless.