WIP: Use Sequel Delayed Job plugin

Tests are passing locally but we still need to run the yetis against an
instance using this Cloud Controller code
Fix issue with requiring delayed job before migrations occur
Adds a rescue for EISDIR errors deleting droplets

This occurs only on local blobstores where the old droplet key format
is also the path of the parent directory of the new droplet key
format. This means if there are multiple droplets associated with an
app (for example if the app is ever updated) then deleting the app
locally results in trying to delete a non-empty directory, which
fails. Fixed tactically in this commit by adding a rescue, we intend
to fix this more fully in the future by properly cleaning up old
droplets at an appropriate time.