PresenceChannel configuration is cached using redis. That cache is used, and sometimes repopulated, during normal GET requests. When the primary redis server was readonly, that `redis.set` call would raise an error and cause the entire request to fail. Instead, we should ignore the failure and continue without populating the cache.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
