Add unique index to Artefact slug field

This has a couple of benefits:

1. It will catch any uniqueness violations that validates_uniqueness_of
misses
2. Small performance benefit:

Times for requesting 5 different artefacts 10 times over in content_api
on my dev VM:

```
          Cold  Warm1 Warm2 Warm3
No Index  3.743 3.697 3.613 3.582
Indiex    3.082 2.817 3.154 3.087
```

The cold column is with a freshly started instance of both mongo and
content_api.
Merge pull request #70 from alphagov/add_artefact_slug_index

Add unique index to Artefact slug field