Add XDEBUG_MODE=coverage to travis descriptor.

Author: GhaziTriki

.travis.yml

@@ -26,7 +26,7 @@ before_script:
 
 script:
   - mkdir -p build/logs
-  - if [ "$TRAVIS_PHP_VERSION" != "nightly" ]; then vendor/bin/phpunit --coverage-clover build/logs/clover.xml; fi
+  - if [ "$TRAVIS_PHP_VERSION" != "nightly" ]; then XDEBUG_MODE=coverage vendor/bin/phpunit --coverage-clover build/logs/clover.xml; fi
   - if [ "$TRAVIS_PHP_VERSION" == "nightly" ]; then vendor/bin/phpunit; fi
 
 after_script: