Try with "composer update" instead of "composer install". The latter does not...

Try with "composer update" instead of "composer install". The latter does not support --prefer-lowest.

.travis.yml

@@ -11,7 +11,7 @@ matrix:
   - php: hhvm
 before_script:
 - if [[ "$TRAVIS_PHP_VERSION" == "5.3" || "$TRAVIS_PHP_VERSION" == "5.4" ]];
-  then composer install --dev --prefer-lowest; else
+  then composer update --dev --prefer-lowest; else
   composer install --dev;
   fi
 script: phpunit --configuration tools/phpunit