Add back SimpleCov

This partially reverts 5e0f412b17aa2faa5e2e71bc11dfa502a7f6ddb7 with the
following differences:

Configure the SimpleCov behaviour so that it 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.

Also, SimpleCov#result.format! is called by default when SimpleCov
exits[1], so there's no need for us to explicitly call it.

The SimpleCov::Formatter::HTMLFormatter#output_message only prints the
'Coverage report generated for' message, so by monkey-patching it we
don't lose the other warnings that we want to keep (mentioned above).