Change SimpleCov behaviour to stop reporting

Change the SimpleCov behaviour so that it no longer reports coverage
when `rake` completes and acts only as a 'red light' if coverage drops
significantly (by 20%) or drops below a percentage (80%).

Note that to silence the 'Coverage report generated for' message that
reports the coverage percentage when SimpleCov completes, we
monkey-patch the SimpleCov::Formatter::HTMLFormatter class.