Update GOV.UK asset host in Travis config

static.production.alphagov.co.uk no longer has DNS, so we should
use the CDN URL instead.
Update Slimmer from 3.20.0 to 5.0.0

https://github.com/alphagov/slimmer/compare/v3.20.0...v5.0.0

Version 3.20.0 of Slimmer is quite old, and tries to fetch things
from static.preview.alphagov.co.uk. This no longer works.