Ensure Greek characters are transliterated
Ensure Greek characters are transliterated

At the moment there's an issue where Greek characters aren't been
transliterated by ActiveSupport, and instead we end up with an empty
string:

```
> "Στερεά Ελλάδα".parameterize
=> ""
```

This is an issue because this output is used the generate the slug of
documents. After this change we get a more usable output:

```
> "Στερεά Ελλάδα".parameterize
=> "sterea-ellada"
```

I got the letters from https://gist.github.com/Achillefs/925267 and I'm
not sure about the validity of them.

This has come up as part of the "Protected Food & Drink Names" finder
where some titles are composed entirely of Greek letters and aren't able
to be saved correctly because the slug ends up being an empty string.

alphagov/specialist-publisher,alphagov/specialist-publisher,alphagov/specialist-publisher