Update to support multiple browsers in CI tests

- Add support for running IE and Firefox
- Update to use build-prod for testing against
- Update the prod config.js to use the sandbox by default. It's not used in
the charm deploys and is safe for us to change the default behaviour of
without issue.
- Update the process doc since we've tweaked the prod config.js settings.
- Make the test running verbose in sauce tests to aid in tracking and
debugging.
- Update browsers we run against to be the latest firefox and current sauce
supported windows.