Add instructions for `saucelabs` reporter to the README

Includes https://github.com/karma-runner/karma-sauce-launcher/pull/28 to fix issues with the reporter.

Fixes https://github.com/karma-runner/karma-sauce-launcher/issues/27.

The reporter is also added to the example used on Travis. However, it doesn't cause the build to fail if the reporter fails - it is simply logged to the build output.