fix Symfony builds

Author: xabbuh

.travis.yml

@@ -30,7 +30,7 @@ matrix:
 before_install:
     - if [ "$COVERAGE" != "yes" -a "$TRAVIS_PHP_VERSION" != "hhvm" ]; then phpenv config-rm xdebug.ini; fi
     - composer self-update
-    - composer require symfony/framework-bundle:${SYMFONY_VERSION} --no-update
+    - if [ "$SYOMFONY_VERSION" != "" ]; then composer require symfony/framework-bundle:${SYMFONY_VERSION} --no-update; fi
 
 install:
     - composer install --prefer-dist