Add support for UUIDs as keys

[chiperific]

lib/fixturebot/key.rb generates integers for primary keys, but a lot of apps are using UUIDs.
I'd like to work on a PR that adds support for UUIDs while maintaining stable IDS using [UUID v5](https://en.wikipedia.org/wiki/Universally_unique_identifier#Versions_3_and_5_(namespace_name-based)