Completely disable exception debug pages in prod

Because of Rails default behaviour of showing debug pages to trusted IPs in
production, combined with external requests appearing to come from internal
IPs, we were showing debug pages on error to most requests.
Override default whenever Rake/Runner task

We need the environment to be set up the way the apps have it, i.e via
`govuk_setenv`. This overrides the default Rake and Runner tasks within the
schedule.rb itself - it may be better to do this in an initialiser so we
can vary based on deploy.
Merge pull request #148 from alphagov/disable_debug_pages_in_production

Completely disable exception debug pages in production
Merge pull request #149 from alphagov/govuk-setenv-scheduled

Override default whenever Rake/Runner task
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