enforce minimum code coverage requirement before merging to master

[donrestarone]

calculate current test coverage after we fix: https://github.com/restarone/violet_rails/issues/245

and then add a rule that prevents merging to master if the change lowers coverage

[donrestarone]

to see test coverage report please run:

./clean_run_tests.sh

and then you will see

open coverage/index.html

[donrestarone]

CI example: https://github.com/restarone/violet_rails/blob/master/.github/workflows/ruby.yml

[donrestarone]

if test coverage is lower with the change applied, we should fail