Bump symfony/form minimum version

Author: GuilhemN

composer.json

@@ -24,7 +24,7 @@
     "require-dev": {
         "symfony/class-loader": "~2.1|~3.0",
         "symfony/yaml": "~2.1|~3.0",
-        "symfony/form": "~2.1|~3.0",
+        "symfony/form": "~2.3|~3.0",
         "willdurand/propel-typehintable-behavior": "^1.0.4",
         "propel/propel1": "^1.6.5",
         "phing/phing": "~2.4",