Remove hardcoded database connection from spec_helper.

This is instead loaded from spec/database.yml - an example
file is provided. This allows different people to run tests
on their own redshift clusters.

In addition, tests have been moved under a test-specific
schema - this needs to be created before runnning tests on
your cluster.
Ensure INSERT RETURNING is disabled in latest Sequel version.

The latest (4.13 as of writing) version of Sequel doesn't seem
to pay any attention to supports_returning? anymore. There is
a method/opts value called disable_insert_returning that has
been added back in - just ensure that this is set all the time
on a dataset.