Datakitten is pinned to an old version

[Floppy]

Updating Datakitten brought in the DataKitten::Fetcher class, which doesn't serialise correctly when used with Marshal in https://github.com/theodi/csvlint/blob/master/app/models/package.rb#L26. Using Marshal there is a bad idea anyway, and should be nuked from orbit, so we've just pinned Datakitten back a ways until that's removed and replaced with something sane.