Replace UUIDTools with SecureRandom.uuid

The library uses the outdated UUIDTools gem to generate UUIDs. However, Ruby ships with a UUIDv4 generator in the standard library: `SecureRandom.uuid`. 

I found two different UUIDs being used:
- random (v4) - `SecureRandom.uuid` is a direct replacement for those.
- timestamp based (I think v1 but I'm not sure). These are used as client tokens. After checking it with the API docs, a client token just has to be unique, and there's no specific requirements for it. So `SecureRandom.uuid` will work just fine.

SecureRandom is implemented natively, so it's faster, and this change eliminates an unnecessary dependency.