Fix cql-rb example

The example uses private APIs (`Ione::CompletableFuture`) and present an async API when cql-rb's primary API is synchronous. `Client#close` also didn't work.

This changes the example to be synchronous.

I can provide a wrapper for the async API too, but to help with porting cql-rb applications this one will work better. It's not complete (it's missing `Client#use`, `PreparedStatement#batch`, for example), but it's probably good enough that people can add what they need themselves, it's just an example after all.