Adding prefer lowest tests in Travis

Author: moufmouf

.travis.yml

@@ -5,8 +5,13 @@ php:
  - 7.0
  - 7.1
 
+env:
+  matrix:
+  - PREFER_LOWEST=""
+  - PREFER_LOWEST="--prefer-lowest"
+
 before_script:
- - composer install --no-interaction
+ - composer install --no-interaction $PREFER_LOWEST
 
 script:
  - vendor/bin/phpunit