Update to latest rails patch release

Fixes:

* The mechanics of setting ordering on document collection groups has
had to change slightly because of "bug fixes" in ActiveRecord 3.2.15.

* Units passed to number_as_currency helper marked as html safe

* Get rid of deprecation warning by being explicit about I18n locale
enforcement (maintains existing behaviour)
Merge pull request #1117 from alphagov/rails-3-2-16

Update to latest rails patch release