Uppercase gov.uk to GOV.UK

In line with branding guidelines. Capped up because we mean business.
Upcased two references to GOV.UK

In line with branding guidelines. "Capped up because we mean business", as I recall.
Merge pull request #294 from alphagov/uppcase-gov-uk-on-shares-asset-libraries-page

Uppcase gov uk on shares asset libraries page