exception_notification submodule uses deleted repository

[DNCrane]

The exception_notification submodule requires https://github.com/cainlevy/exception_notification, which seems to have been deleted. I managed to get everything working by removing the submodule and installing a separate version (https://github.com/rails/exception_notification/tree/2-3-stable) of exception_notification, but I don't have enough knowledge of Rails or Ruby to confidently fix this issue for others.