Add link to test coverage report to README

What does this MR do?

Since we have GitLab Pages with test coverage report, it makes sense to link coverage badge there, instead of linking to commits page.

Reassigned to @axil

What do you think about it @axil? 😸

@axil One thing to note is that we actually have same README for GitLab EE, which can be confusing because we have http://gitlab-org.gitlab.io/gitlab-ee/coverage-ruby as well. But this is probably not worth introducing a conflict in README, so it may be good idea to default to CE.

Mentioned in issue gitlab-com/blog-posts#272 (closed)

Oh yes!

Enabled an automatic merge when the build for 7336e73b succeeds

Canceled the automatic merge

Canceled the automatic merge

Status changed to merged

Mentioned in commit a5cd9c9a