Sort artefacts for a tag in a case-insensitive order

Ruby's sort_by is case-sensitive, so a typical result would sort all the 
artefacts with initial uppercase characters higher than those without.

This isn't a case often seen in Mainstream content, but it's a common 
problem with detailed guides with a subject prefix (eg. "Oil and gas: 
access to data" vs "Oil and gas: DECC Public Register")
Merge pull request #151 from alphagov/case-insensitive-sorting

Sort artefacts for a tag in a case-insensitive order