Upgrade Express to 4.4.1

Want strong ETags.

See https://github.com/visionmedia/express/wiki/Migrating-from-3.x-to-4.x

Connect is no longer part of Express, so middleware is no longer on the
`express` object. Instead, we import the things (occasionally with new
names) and use those.
Merge pull request #559 from alphagov/upgrade-express

Upgrade Express to 4.4.1