Switch from Rack::MailExceptions to the exception_notification gem

Rack::MailExceptions uses TMail so doesn't work with Ruby 1.9.3 and is
no longer maintained.

exception_notification seems to be the most actively developed gem in
this area so I've decided to use it despite it bringing in lots of
dependencies and being quite fiddly to get up and running.
vcr is only known to work up to webmock 1.11