use phpunit flags

.travis.yml

@@ -1,5 +1,10 @@
 language: php
 
+cache:
+  directories:
+    - vendor
+    - $HOME/.composer/cache
+
 git:
   depth: 2
 
@@ -13,15 +18,16 @@ before_script:
   - composer install --prefer-source --no-interaction
 
 install:
-  - if [[ $COVERAGE == true ]]; then composer require php-coveralls/php-coveralls; fi
+  - if [ $COVERAGE ]; then composer require php-coveralls/php-coveralls; fi
+  - if [ $COVERAGE ]; then PHPUNIT_FLAGS="--coverage-clover build/logs/clover.xml"; fi
   - composer require jakub-onderka/php-parallel-lint
 
 script:
   - mkdir -p build/logs
   - vendor/bin/parallel-lint --exclude vendor .
-  - vendor/bin/phpunit --configuration phpunit.xml --coverage-clover build/logs/clover.xml
+  - vendor/bin/phpunit $PHPUNIT_FLAGS
 
 after_success:
-  - if [[ $COVERAGE == true ]]; then travis_retry php vendor/bin/php-coveralls -v; fi
+  - if [ $COVERAGE ]; then travis_retry php vendor/bin/php-coveralls -v; fi