configure phpunit flags

.travis.yml

@@ -64,7 +64,7 @@ before_script:
 
 script:
   - composer validate --no-check-lock
-  - vendor/bin/phpunit -v -c phpunit.xml --testsuite only_fast_tests --coverage-clover=coverage.clover
+  - vendor/bin/phpunit ${PHPUNIT_FLAGS}
 
 after_success:
   - if [[ $COVERAGE = true ]]; then wget https://scrutinizer-ci.com/ocular.phar; fi