Set xdebug coverage mode to generate coverage report

Author: wol-soft

.travis.yml

@@ -29,7 +29,7 @@ before_script:
   - mkdir -p build/logs
 
 script:
-  - ./vendor/bin/phpunit --coverage-clover build/logs/clover.xml --testdox
+  - XDEBUG_MODE=coverage ./vendor/bin/phpunit --coverage-clover build/logs/clover.xml --testdox
 
 after_script:
   - ./cc-test-reporter after-build --exit-code $TRAVIS_TEST_RESULT