Flip `unless...else...end` statement.

It's easier to understand as an `if...else...end` statement,as suggested
by our style guide
<https://github.com/alphagov/styleguides/blob/6ff2832be055/ruby.md>.
Merge pull request #89 from alphagov/refactor-needs-duplicate-method-usage

Flip `unless...else...end` statement.