remove guard around exception notification

since exception_notification was upgraded to v4.0.0, the 'feedback'
app hasn't been sending emails on error because the guard should have been
(but wasn't) updated.

I am not able to think of a scenario where it would be useful, so removing it.
update to new exception_notification 4.0 syntax

in order to silence deprecation warnings
Merge pull request #67 from alphagov/adjust-exception-notifier

Exception notifier bugfix