updated to work with most recent sequel gem (4.9.0)

The work for this was pretty minor:
1. Change String#lit to Sequel#lit
2. Remove Database#default_schema= nil from spec (default_schema has been removed from Sequel)
3. Give the fully qualified name of for table_name in a join (lib/sequel/adapters/vertica.rb:106)
4. Add :core_extensions, since the methods #as et. al were moved out of Sequel::Core a while back
5. Sequel::Model.cache_anonymous_models = false --> Sequel.cache_anonymous_models = false

All and all, the while pull request is only a few lines of changes (plus killing some bonus whitespace)