Fix create capped collection

`db` is now a backdrop Database which has a slightly different signature
for creating a capped collection. This only manifests itself when the
collections do not already exist.
Add migration to convert GA timezone to UTC

We are ignoring the timezone from GA requests so previous data needs to
be converted to account for this.

See https://www.pivotaltracker.com/story/show/62779118 for details.
Merge pull request #220 from alphagov/fix-ga-bucket-timezones

Add migration to convert GA timezone to UTC