Adding prefer lowest tests in Travis

Author: moufmouf

.travis.yml

@@ -11,7 +11,7 @@ env:
   - PREFER_LOWEST="--prefer-lowest"
 
 before_script:
- - composer install --no-interaction $PREFER_LOWEST
+ - composer update --no-interaction $PREFER_LOWEST
 
 script:
  - vendor/bin/phpunit