Fix for cases not being sorted by updated_at

Previously when no keyword was applied, cases were in a random order. Now,
cases are sorted by updated at unless there is a keyword.

Relates to https://trello.com/c/FXFzVQs5/67-bug-cases-not-in-last-updated-order-for-cma-finder
Merge pull request #20 from alphagov/fix-ordering-of-cma-cases

Fix for cases not being sorted by updated_at