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 #149 from alphagov/govuk-setenv-scheduled

Override default whenever Rake/Runner task