Update appraisal gemfiles

Test against the latest rails-master and stripe-ruby
Refactor internal usage of AS::Notifications

These changes are inspired by webmachine-ruby's integration
with AS::N.
https://github.com/seancribbs/webmachine-ruby/blob/6edaecea1ceab7c2c54fc5e83446317524f6bb8b/lib/webmachine/events.rb
Reference the described class within specs
Push event retrieval into lib
Remove screenshots directory
Run specs against the latest