Add missing require to active_storage.rb

Since b21f50d, requiring active_storage
on its own has failed with the following error:

    activestorage/lib/active_storage.rb:55:in `<module:ActiveStorage>': undefined method `minutes' for 5:Integer (NoMethodError)

activestorage/lib/active_storage.rb

@@ -26,6 +26,7 @@
 require "active_record"
 require "active_support"
 require "active_support/rails"
+require "active_support/core_ext/numeric/time"
 
 require "active_storage/version"
 require "active_storage/errors"