added PHP_ENABLE_XDEBUG test

Author: schmunk42

.travis.yml

@@ -55,6 +55,7 @@ script:
   - docker-compose build --build-arg X_LEGACY_GD_LIB=$X_LEGACY_GD_LIB
   - echo "travis_fold:end:BUILD folding ends"
   - docker-compose run --rm php php -v
+  - docker-compose run --rm -e PHP_ENABLE_XDEBUG=1 php php -v
   - docker-compose run --rm php php /tests/requirements.php
   - docker-compose run --rm -w /yii2 php composer install --prefer-dist
   - docker-compose run --rm -w /yii2 php php -d error_reporting="E_ALL ^ E_DEPRECATED" vendor/bin/phpunit tests/framework/ --exclude db