[Bridge/Twig] Fix lowest form dep

Author: nicolas-grekas

src/Symfony/Bridge/Twig/composer.json

@@ -22,7 +22,7 @@
     "require-dev": {
         "symfony/asset": "~2.7",
         "symfony/finder": "~2.3",
-        "symfony/form": "~2.7,>=2.7.6",
+        "symfony/form": "~2.7.8|~2.8,>2.8-BETA1",
         "symfony/http-kernel": "~2.3",
         "symfony/intl": "~2.3",
         "symfony/routing": "~2.2",