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.