Update db:migrate script, database.yml, spec setup

The gem shouldn't conflict with the default use cases
and if you don't set a schema and table name, your
application will not be able to create the necessary tables.
Bump version, update sequel-vertica, gem dependencies

We need to be able to bump sequel to get bug and performance
fixes. Since sequel-vertica doesn't seem to be actively
maintained, we can have our own version to do updates.