fix(coverage): use Simplecov instead of Coverall

[iberianpig]

Remove Coverall that has errors in Github Actions. Use Simplecov for check coverage instead of Coverall.

It shows coverage when running RSpec in the developer's local environment.

$ bundle exec rspec

....
Finished in 0.45914 seconds (files took 0.56276 seconds to load)
189 examples, 0 failures, 7 pending

Coverage report generated for RSpec to /home/iberianpig/.ghq/github.com/iberianpig/fusuma/coverage. 2162 / 2322 LOC (93.11%) covered.

Then, open the result of coverage on the browser

$ open ./coverage/index.html

closes: #270