Downgrade simplecov to 0.17 because 0.18 is incompatible with code climate. Ref: github.com codeclimate/test-reporter/issues/413
* Fix simplecov clobbering test coverage reports when using parallel tests. * Generate coverage reports by default (remove $SIMPLECOV flag). * Store coverage reports in tmp/coverage/ instead of coverage/. * Enable branch coverage. ref: github.com /colszowka/simplecov/issues/718#issuecomment-538201587