Let AR::Base properly lazy-loaded if Railtie is available

Since the current code calls `ActsAsList::Railtie.insert` in the top-level, just requiring this gem (usually via Bundler) would immediately load AR::Base and trigger all of its initializers, which would slowdown the server startup, spec execution that does not depend on AR, etc.

Also, currently `ActsAsList::Railtie.insert` call happens three times on the gem initialization, which of course is actually needed to be done only once.