Do not implicitly set up rack-timeout middleware

By requiring `rack-timeout`, `Rack::Timeout` is automatically added to the middleware of a Rails application. 

See https://github.com/heroku/rack-timeout#rails-apps

This leads to unexpected behavior. When a user adds a non-standard `Rack-Timeout` to the middleware stack herself, she ends up with `Rack-Timeout` added twice. Worse, the configuration added automatically is the standard configuration which might differ from the one added manually in user land. Worst case comes to show when a user configures a `service_timeout` greater than the standard one.

Author: janraasch

lib/raven/integrations/rack-timeout.rb

@@ -1,6 +1,6 @@
 # rubocop:disable Style/FileName
 # We need to do this because of the way integration loading works
-require 'rack-timeout'
+require "rack/timeout/base"
 
 # This integration is a good example of how to change how exceptions
 # get grouped by Sentry's UI. Simply override #raven_context in