[2.6] Test lowest versions of dependencies

Author: nicolas-grekas

composer.json

@@ -32,9 +32,9 @@
         "symfony/twig-bundle": "~2.2",
         "symfony/twig-bridge": "~2.2,>=2.2.6",
         "symfony/process": "~2.0,>=2.0.5",
-        "symfony/validator": "~2.2",
+        "symfony/validator": "~2.5",
         "symfony/yaml": "~2.0,>=2.0.5",
-        "symfony/expression-language": "~2.4",
+        "symfony/expression-language": "~2.6",
         "doctrine/doctrine-bundle": "~1.2",
         "twig/twig": "~1.12"
     },