Tidy up coveralls integration

[mattwynne]

Today we discovered a formatter class with no unit tests. There was a bug around this formatter, and the lack of unit tests made it harder to fix.

Measuring code coverage from unit tests would have helped us to catch this problem sooner.

• fix coverage badge on README
• add a bot (like in CucumberJS) to comment on PRs about the impact on code coverage

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in two months if no further activity occurs.