Rename "industry_sectors" to "specialist_sectors"
Add migration to update existing industry sector tags

This adds a migration to update tags with the "industry_sector" type to 
now have the type "specialist_sector".
Bump govuk_content_models to.7.0.0

This includes the renaming of industry_sector to specialist_sector in 
the list of an artefact's tag types.
Merge pull request #126 from alphagov/change-industry-sector-to-specialist-sector

Change industry sector to specialist sector

https://www.pivotaltracker.com/story/show/65625746