minor #1277 Fix comment about symfony version (Tobion)

This PR was merged into the 1.3 branch.

Discussion
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Fix comment about symfony version

Form FQCN are introduced with symfony 2.8.

Commits
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ff161a9a Fix comment about symfony version

src/Silex/Application/FormTrait.php

@@ -31,7 +31,7 @@ trait FormTrait
     public function form($data = null, array $options = array())
     {
         $name = 'Symfony\Component\Form\Extension\Core\Type\FormType';
-        // for BC with Symfony pre 2.7
+        // BC with Symfony < 2.8
         if (!class_exists('Symfony\Component\Form\Extension\Core\Type\RangeType')) {
             $name = 'form';
         }