Bump govuk_content_models to 6.0.3

This adds the new tag type of 'industry_sector' to the Artefact model.
Allow industry_sector tags to be given for artefacts

This adds a functional test for setting industry_sector tags on an 
artefact, and adds the 'industry_sectors' attribute to the whitelist of 
fields which can be provided in the top level of the params hash.
Add migration to create oil and gas sector tags
Bump govuk_content_models to 6.0.5

This fixes a bug where double dashes inside an Inside Gov slug were 
changed to a single dash.
Merge pull request #110 from alphagov/create-oil-and-gas-tags

Prepare for and create Oil & Gas industry sector tags