Test coverage in code climate

This PR will make test runs on travis push test coverage stats to code climate, so code climate can show both code quality metrics and test coverage side-by-side.

I've also tidied up the way we require dev dependencies, so that the same dependencies are loaded in all test runs on travis (previously the test runs using any of the Gemfiles in `./gemfiles/` were not loading minitest 5 or rake, apparently.