Use UTC dates

As per
https://github.com/alphagov/external-link-tracker/commit/16b2cb2239c71568f2a8e66914051d376d7513d8#commitcomment-5495391,
we should ensure we're storing hits in UTC only.
Wrap time.Now() so we can mock it

This allows us to force Now() to use a specific value so we can be
assured that the value stored in the database is the value we want.
Fix non-deterministic test ordering

The tests were failing prior to this because they depended specifically
on the date being returned correctly. Because we were using the same
hostname for all tests we would only randomly get the correct data for
the test we're looking at.

Lessons learned:

- Go executes tests in parallel by default
- Mongo doesn't provide us with any tools to help here
- setUp and tearDown functionality that was executed in a guaranteed
  order would have helped
Merge pull request #2 from alphagov/use-utc

Use UTC dates