Bump content models to 5.1.0

This relaxes the uniqueness constraint on LocalAuthority#local_directgov_id.  These sometimes swap around upstream, and then cause the importer to fail.  There's no need to enforce this here, as it's already done upstream.
Merge pull request #106 from alphagov/fix_local_directgov_uniqueness

Relax uniqueness constraint on local directgov id