設定 xdebug 的 mode 為 coverage

關聯的失敗 job
- https://travis-ci.org/github/CQD/qlurk/jobs/753770091

關聯的 phpunit issue
- sebastianbergmann/php-code-coverage#834

Makefile

@@ -9,4 +9,4 @@ installWithDev:
 	composer install -o --prefer-dist
 
 e2e: installWithDev
-	./vendor/bin/phpunit -c e2e-test/phpunit.xml --coverage-text
+	XDEBUG_MODE=coverage ./vendor/bin/phpunit -c e2e-test/phpunit.xml --coverage-text