Add back SimpleCov

This partially reverts 5e0f412b17aa2faa5e2e71bc11dfa502a7f6ddb7 with the
following differences:

Configure the SimpleCov behaviour so that it doesn't report 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).
Pin SimpleCov to version 0.7.1

Pin SimpleCov to version 0.7.1 to avoid the bug in SimpleCov 0.8.x
affecting exit codes:

colszowka/simplecov#281

Note that I had to change `profiles` to `adapters`, as the naming
changed in version 0.8.0[1].

[1]:
colszowka/simplecov@60f4717#diff-4ac32a78649ca5bdd8e0ba38b7006a1eR16
Adjust SimpleCov thresholds

Set the minimum coverage to 99% to match the current coverage, as
discussed with @annashipman.

Also, remove the maximum coverage drop as this will never be triggered
given the minimum coverage we have set.
Merge pull request #45 from gds-operations/72169016-simple-cov-red-light

[#72169016] Add back SimpleCov