Increase cache duration for Spotlight assets

I'd like to increase the Expires header as an indication that I'm more
confident in our ability to cachebust than I was previously.

Currently the only assets that won't be cachebusted are images in stylesheets
and the govuk_template - the govuk_template will be cachebusted as soon as
https://github.com/alphagov/govuk_template/pull/50 is merged and we update
to it.

We're still seeing some odd issues whereby the page is rendered with no CSS.
I think this is because the browser caches the HTML for half an hour which then
references CSS that no longer exists. I think we can solve this by increasing the
Expires header further and putting the assets vhost behind Varnish.
Merge pull request #219 from alphagov/increase-cache-time

Increase cache duration for Spotlight assets