Relax uniqueness constraint on local_directgov_id

We're not actually using this field, and the id's sometimes swap round
upstream, which causes errors on import.  We can relax this, and rely on
the uniqueness being constrained in the upstream data.
Merge pull request #74 from alphagov/relax_local_directgov_id_uniqueness

Relax uniqueness constraint on local_directgov_id